If puck likes her why does he alawys prank her?

Because he likes her! Boys around that age (11 or 12-ish) always tease and prank the girls they like. Of course he doesn't want her to figure out that he likes her, so he pranks her even more than he pranks other people. Plus, he's Puck. He's been pranking people for thousands of years, and he is most certainly not going to stop now! :D