At the end of each Theology course, we call the students forward at Sunday worship and the pastor hands them a nice certificate for completion of the course, congratulates them, and encourages others to sign-up for the next course. During the course itself, I emphasize again and again that in order to receive a certificate, all assigned homework must be turned in. After teaching 75 students in various courses, everyone has completed their homework. On occasion the assignments were handed in late, but they received credit none-the-less.
That's a great idea to hand certificates during Sunday worship...brings some publicity to the course and gives some recognition for the students' work.
That is great that you have been able to take 75 students through!