I can only speak from experience.
When I was a beauty school student, it was hard when a client did not tip me. I always did a great job and won various awards. Student stylists don't make any money no matter how long or how hard they work on a client, so it's always a very nice thing to receive a tip for a job well done.
I remember doing foils on a few of my clients, two in particular. These ladies are Brazilian and had very thick hair that fell to brastrap. One had so much hair that partial highlights on her equated to 80 foils! If you don't know, let me be the first to tell you that 80 foils is a LOT. Her entire service, including shampoo and styling, was only $30. She never once tipped me. And she was coming to see me every four-five months by request, so she was pleased with my work.
The prices are already low, for crying out loud! (Sorry, just a little residual frustration there, not directed at anyone here.
) In all, those who tipped kind of made up for those who didn't.
I commend you for being willing to tip your student stylist, and for thinking ahead. 10-20% is a good rule of thumb, depending on the service total and how satisfied you are with the service.
ETA: chances are good that only one student will work on you from start to finish, so no need to tip more than one person.