This is the first time i've ever posted here. And it's because I'm going through something like this right now. Im just a little farther along. I'm 18 and a senior in High School. I ruined things with my best friend last month by thinking she liked me after some (In what I though was) flirtation. We barely talk now.
No offense to any of the people giving advice to you, but they are all a lot older than high school kids. Things are different now. Teenagers are a lot more comfortable with their bodies and making sexual jokes/flirtation and they see it as harmless.
In my experience we were doing things that sound like how you two are acting. We are both in the tech crew (Behind the scenes of theater) The tech crew in my school has a reputation for being pretty raunchy and wild. Pantsings, grabbing, and stuff like that. Me and my best friend were getting pretty close, i made a move and now we dont even talk.
Im not tellin you wut to do. Part of the situation is different but parts are the same. Just thought that you might want to hear what happened when I did what you're thinking about doing.