Pick your girl, next time get to talk with her, have a laugh, tease her a bit and put your arm around her and give a squeeze, comforting her after making a joke.
But don't take your arm away just lighten up the touch and let your arm rest on her hip lightly, when working on a social circle I try and keep constant contact as much as possible, as long as it's not completely awkward, but if you act like it's normal they will accept it as being normal too.
Tease and joke, build up sexual topics and aim it in the direction between you both, pretend to kiss her or something, then if she reacts badly you can make out you were just joking around and she needs to lighten up, just cause you were talking to her doesn't mean you were hitting on her.
If scared of getting caught out flirting, flirt in a fun joky manner so you can eliminate the reality if needed.
Most important thing is just to keep things comfortable and fun don't heavy flirt and risk creeping her out a bit, in a school environment you're right it's not the road you want to go down
I'm new to giving advice for these things to be honest, in my head I know how I would act and stuff but not always sure how to explain to people.