When I was in middle school I was the drummer/backup vox for a ska band with a bunch of my older friends(college guys). Probably the best experience I've had so far, just playing people's birthdays and random small hole-in-the-wall type places. I'm actually starting up a new band and all the acoustic stuff we're going to do is going to include my uke.
I used to jam with my best friend in high school and it was definitely fun. We played for our friends. If you want to go public, try composing a couple songs or learn some fairly popular songs and cover them as a group. Once you have a couple things set up just invite people to like, jam session type things in places where you're welcome at. Local coffee shops, bars, schools, etc etc. If you guys somehow make it part of the school program or something you could use that for your college resume. Looks really good too, haha.
The worst that can happen is you go out and embarrass yourself. But whatever, like Every Time I Die put it, "I'd rather live my life in regret than not take a part of it". You also get to tell that story for a long long while.
