How much to tip at a hair school?

I'm curious how much should be tipped at a hair school? My local college has a Cosmetology program, since I'm a student I can receive free hair cuts. I went in a few weeks ago and got a great cut for free but tipped $5.00. The stylist was great and told me to come in and get color done from him, so today I did. He did an incredible job. The bill was $18.00 and I tipped another $5.00. He didn't seem thrilled when he seen the $5.00. Now I feel bad, like maybe I should have tipped more. I did ask the girl that was doing hair beside him what the average tip was and she said $1 - $3. She had just done a perm and cut and only got $3. Should I have tipped more?