Attendance and invoice database design


I am designing a database for managing a small private school (~15 teachers, 250 students). The student and teacher contacts, lessons and attendance I think I got it covered (see the image).

The problem I am facing now is the following:

The students pay in advance for 10 or 30 lessons. I then need to keep count of every lesson the student followed. Once they have no more lessons left, have a notification so that the student can pay for other 30 (or 10) lessons and then track again.

I sure can count how many lessons the student followed and compare with how many payments the student has done (eg 57 lessons, 2 payments => 3 lessons left) but is that a good strategy?

I also need to have a monthly track of how many students a teacher had.

Any advice?

Thanks!

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