You could have it as part of your grading system that all students at the different levels have to have attended a certain amount of seminars to progress.
This of course would work both ways, it would mean you have to do more and more have to attend.
I have practised in a dojo that actually does this. People have left because of this. They said the dojo became commercial and had moved away from traditional ways of teaching an art
People could only do exams on specific saturdays (seminar), for which a fee had to be paid. So you had to pay to attend the seminar and
you paid for the exam. Sure this brings in the dollar (or euro in our case)...