PHP - How to only post checked checkboxes in a loop?

I am working on a project that signs up students to specific classes("french", "math",...). The student id is already selected. True a while loop i display my classes out of my database with a checkbox in front of each class. I only wanna post te checked checkboxes in a form that will send it to a php file thats puts the student and his selected classes into the database. But i cant seem to solve how i only post the checked checkboxes because of the difficulty with the while loop. I hope someone can help me. Already thanks!