Of course, development is uneven. A child may be ahead in math, average physically, and behind in reading - and that's normal. Or they might write like a champ, read kinda okay, suck at math, and be a whiz socially. Great. Maybe they have a great grasp of logic, well above what you should expect - shouldn't that be rewarded? It's a valuable academic skill.

If you can't cut class sizes a lot smaller, in groups that are closer in age, seems the solution might be to have classes that span two, or even three years, and you stay with the same teacher for that time. So if you're a year ahead of your age in one subject, and a six months behind in another... well, it should average out.