>average they would do better the next year because of reversion to the mean.

I don't think it works like that. Students who do poorly often do WORSE the next year. They tend to get more and more behind and eventually just drop out.

The only way to reverse this trend is to identify them for more academic instruction, like an extra math or reading class.

Even then, it often doesn't work. It's really really hard to turn a low performing student into even an average one.

"Reversion to the mean" has no meaning when you are talking about one student from one year to the next.