It won't affect weekly grosses much. It mainly just changes the weekend/weekday distribution. So weekend drops will go a bit lower than they usually would if schools weren't starting up, while weekday drops get a bit bigger.
Let's say a movie would usually drop 30% from one week to the next and it gets 60% of its box office on the weekend each week with schools out. If many more schools are in session the next week, that could make it get 70% of its box office during the weekend that week.
That would mean that instead of 30% weekday drops and a 30% weekend drop, it gets a 18.3% weekend drop and a 47.5% weekday drop to maintain a 30% weekly drop
(The change won't be this extreme in one week btw, this is just a random example to show my point).
That's not bad! I figured it would find a way to balance itself out