It's possible that the amount of licenses that you want to add is superior to the one you have. You will be able to know when you try and add more registrations by the number on the upper-left side, it indicates the total amount of licenses you have at disposal still.

If this is your case, contact your school License Manager, for them to increase the number of licenses they've allotted to you. You may also, delete older classrooms, to release those licenses so you may use them.