I've been plugging BYL this past day as a secular, all-in-one homeschool option that offers unit studies (so, appropriate for short stints at home) and it seems they're doing their spring sale right now. It's a little weak on writing - if you're concerned about that, I'd supplement with Killgallon (if you're most interested in creative writing - I picked for my rec due to the price) or The Writing Revolution (if you're not - I really recommend this anyway.) Or you can do Bravewriter if you want your kid to have online interaction.
https://buildyourlibrary.com/spring-sale/
Math Mammoth (which is suitable through pre-algebra) is also having a sale until the 20th. (From personal experience, I suggest that whatever level your kid tests into - and definitely do the placement test! - you start off doing only half the problems per page.)
They also offer free practice pages. I can say quite honestly that despite my generally haphazard approach to middle school, we went through this curriculum page by page and the kids did just fine once returned to school.
https://www.mathmammoth.com/
This is free, but it assumes a UK scope and sequence of math... or maths, even: http://www.cimt.org.uk/projects/mep/index.htm
You probably don't want to jump into a full history curriculum mid-year, but if you want to start in on geography I can also recommend Mapping The World With Art. Combine that with CNN-10 for current events (suitable for middle school upwards) and you have the basics of social studies.
https://ellenjmchenry.com/product/mapping-the-world-with-art-digital-download/
https://www.cnn.com/cnn10
If you're dead-set on history for the high school or advanced middle school level, but don't want to shell out for it:
https://whfua.history.ucla.edu/
https://school.bighistoryproject.com/bhplive
https://ushistory.org/
https://sheg.stanford.edu/history-lessons
Reading comprehension worksheets on various levels, including prose, history, science, and poetry, and social studies topics:
http://readworks.org/
Penmanship worksheets (you can customize your text):
https://www.kidzone.ws/cursive/
Times table worksheets:
https://www.math-aids.com/Multiplication/Multiplication_Tables.html
This information is intended for people who find themselves stop-gapping a few months in (mostly) grades 1 - 9. I don't include science because we did Building Foundations for Scientific Understanding and that's actually a real pain when it comes to teacher prep. Comprehensive, though! Even though it says "k - 2" it's advisable to start there no matter what grade level your kid is. I suggest you google "science experiments" and go from there - definitely do not buy any curriculum or unit study on biology or geology unless you've double checked that it's not some YEC deal.
Edit: If you're about to say that learning is more meaningful and better when you release yourself from the tyranny of worksheets and/or unschool entirely, you're probably right, but I'm going to assume that the people who want to do that are already doing that. This is for people who just want their kids not to lose any of their academics between now and whenever local schools open again.
If you have other free or inexpensive resources that you trust - that is, you've used them or somebody known to you has used them and recommended them! - please share them.
Edit again: I assume nobody here has to start teaching reading from scratch RIGHT NOW, but if you are in that boat you probably want a synthetic phonics program - and no, your local school almost certainly doesn't use one, they do this half-assed mix of mostly-not-phonics. People generally advise that Spalding is cheap-but-not-fun, and that's accurate. If you use Spalding, definitely use these modifications: https://www.ontrackreading.com/perspectives/modifying-the-spalding-phonogram-set
You might prefer All About Reading or Logic of English. If you suspect a learning disability then these might all be a bit too front-loaded - try Dancing Bears for reading and the companion Apples and Pears for spelling. No joke, Apples and Pears is why Eva can spell well enough for spellcheck to be of any use: https://www.soundfoundations.co.uk/product/dancing-bears-a/
For vocabulary at an upper elementary or middle school level, you can try Red Hot Root Words. That's not a full-year curriculum, but again, I assume nobody here needs or wants one of those.
Edit the thirdish: Apparently The Charlotte Mason Institute is offering a free emergency packet to cover the next few months. I suspect many people reading this might find the Charlotte Mason method appealing, however I'm not familiar with their curriculum in particular and I've found from experience that Charlotte Mason fans tend to be really into old books. Very old books. Books so old you've never heard of them because they're mostly only read by homeschoolers... and despite all their talk about avoiding twaddle, Henty's books aren't any better-written than the stuff coming out today, and they're a lot more racist to boot. The upside of this is that those books are cheap or free. The downside is, well, they're problematic all over.
So if you go for it, double check the reading list: https://charlottemasoninstitute.org/sign-up-emergency-alveary-curriculum/
Edit the fourth: Somebody elsewhere suggested for high school a Great Courses subscription might be enough for just a few months. It's $49 per month, and many libraries have subscriptions already (even if they're closed!) It's not terrific, but it's only a few months: https://www.thegreatcoursesplus.com/
https://buildyourlibrary.com/spring-sale/
Math Mammoth (which is suitable through pre-algebra) is also having a sale until the 20th. (From personal experience, I suggest that whatever level your kid tests into - and definitely do the placement test! - you start off doing only half the problems per page.)
They also offer free practice pages. I can say quite honestly that despite my generally haphazard approach to middle school, we went through this curriculum page by page and the kids did just fine once returned to school.
https://www.mathmammoth.com/
This is free, but it assumes a UK scope and sequence of math... or maths, even: http://www.cimt.org.uk/projects/mep/index.htm
You probably don't want to jump into a full history curriculum mid-year, but if you want to start in on geography I can also recommend Mapping The World With Art. Combine that with CNN-10 for current events (suitable for middle school upwards) and you have the basics of social studies.
https://ellenjmchenry.com/product/mapping-the-world-with-art-digital-download/
https://www.cnn.com/cnn10
If you're dead-set on history for the high school or advanced middle school level, but don't want to shell out for it:
https://whfua.history.ucla.edu/
https://school.bighistoryproject.com/bhplive
https://ushistory.org/
https://sheg.stanford.edu/history-lessons
Reading comprehension worksheets on various levels, including prose, history, science, and poetry, and social studies topics:
http://readworks.org/
Penmanship worksheets (you can customize your text):
https://www.kidzone.ws/cursive/
Times table worksheets:
https://www.math-aids.com/Multiplication/Multiplication_Tables.html
This information is intended for people who find themselves stop-gapping a few months in (mostly) grades 1 - 9. I don't include science because we did Building Foundations for Scientific Understanding and that's actually a real pain when it comes to teacher prep. Comprehensive, though! Even though it says "k - 2" it's advisable to start there no matter what grade level your kid is. I suggest you google "science experiments" and go from there - definitely do not buy any curriculum or unit study on biology or geology unless you've double checked that it's not some YEC deal.
Edit: If you're about to say that learning is more meaningful and better when you release yourself from the tyranny of worksheets and/or unschool entirely, you're probably right, but I'm going to assume that the people who want to do that are already doing that. This is for people who just want their kids not to lose any of their academics between now and whenever local schools open again.
If you have other free or inexpensive resources that you trust - that is, you've used them or somebody known to you has used them and recommended them! - please share them.
Edit again: I assume nobody here has to start teaching reading from scratch RIGHT NOW, but if you are in that boat you probably want a synthetic phonics program - and no, your local school almost certainly doesn't use one, they do this half-assed mix of mostly-not-phonics. People generally advise that Spalding is cheap-but-not-fun, and that's accurate. If you use Spalding, definitely use these modifications: https://www.ontrackreading.com/perspectives/modifying-the-spalding-phonogram-set
You might prefer All About Reading or Logic of English. If you suspect a learning disability then these might all be a bit too front-loaded - try Dancing Bears for reading and the companion Apples and Pears for spelling. No joke, Apples and Pears is why Eva can spell well enough for spellcheck to be of any use: https://www.soundfoundations.co.uk/product/dancing-bears-a/
For vocabulary at an upper elementary or middle school level, you can try Red Hot Root Words. That's not a full-year curriculum, but again, I assume nobody here needs or wants one of those.
Edit the thirdish: Apparently The Charlotte Mason Institute is offering a free emergency packet to cover the next few months. I suspect many people reading this might find the Charlotte Mason method appealing, however I'm not familiar with their curriculum in particular and I've found from experience that Charlotte Mason fans tend to be really into old books. Very old books. Books so old you've never heard of them because they're mostly only read by homeschoolers... and despite all their talk about avoiding twaddle, Henty's books aren't any better-written than the stuff coming out today, and they're a lot more racist to boot. The upside of this is that those books are cheap or free. The downside is, well, they're problematic all over.
So if you go for it, double check the reading list: https://charlottemasoninstitute.org/sign-up-emergency-alveary-curriculum/
Edit the fourth: Somebody elsewhere suggested for high school a Great Courses subscription might be enough for just a few months. It's $49 per month, and many libraries have subscriptions already (even if they're closed!) It's not terrific, but it's only a few months: https://www.thegreatcoursesplus.com/