When setting up a School plan, you choose the number of licenses by selecting how many Classes you're going to purchase.
1 Class consists of 1 Teacher license and 100 Student licenses.
Tip: Not sure how licensing work? Check out Licensing in Explain Everything.
In short, you most probably want the number of classes to be equal to the number of teachers that are going to use Explain Everything in your institution.
Once you decide how many teachers you'd like to give licenses to, the number of Student licenses should no longer be a factor. There should be more than enough Student licenses to allow every student in your institution to use Explain Everything.
In an unlikely scenario where you run out of Student licenses, each Class added to your account will increase the number Student licenses by 100.