I recently tried getting a haircut at a beauty school for the first time, and this particular place didn't allow its students to accept tips. In place of tipping, one had the option of contributing to a scholarship fund instead. I gave around 15% for a $19 cut. I'd suggest calling ahead and asking what is usual or checking out the website if they have one. The place I went had its policy clearly listed on the website.
Also, be prepared for your haircut to take a long time. I'm glad I decided to go to the beauty school and I'll probably go back, but the whole process took about two and a half hours to get 1 inch taken off of my existing style (short, requires almost no styling after cutting). This place includes a couple of complimentary massage/facial sort of things, which takes up some of the time, but most of it was the haircut. First, the student had to make sure she understood exactly what I wanted. Then, she went over the plan with an instructor. Then, she went slowly and carefully, with the instructor checking off after every step of the process and every time she wasn't sure how to proceed. It's a good system; it just takes a long, long time.