I used the form during the summer. But during the school year I will only have 4 transactions a month. Can I downgrade?

Hello, yes, you can downgrade your plan anytime you want. Your forms won't be affected since you're nowhere near the monthly limits of the Free Plan (even after you downgrade).

Here's a guide on How-to-Downgrade-Subscription-Back-to-Free-Account 

And if you need to upgrade again, you can also do that anytime the need arises.

Hope that answered your question.