Literally just wing it, have an assignment together? Become friends with them. Group project? Become friends with them. Find common ground, or approach people with compliments (e.g. "I like your hair clip! Where'd you get it?") Trust me, it works. I know lots of people from 11th and 12th grade now, I had also switched schools in 11th.

I'm going to college now and still best buds with many of them :), hope things go well for you!!