What does it mean? Girls Only?

Sounds to me like he's trying to get your attention, I don't know though whether his intentions are good, please be careful, some teachers take advantage of their students, if you feel that you are being unfairly treated, ask a neutral person, teacher or headmaster etc to review your work and see if they have the same opinion, he could be trying to hit on you so please be careful, goodluck! :)