How do you know if a girl is flirting or friendly?

I'm new to girls and relationships. I used to be really skinny. After eating and exercising a lot more, I put on 20 pounds of muscle over the summer. I'm a high school senior now and it feels like girls are starting to notice me.
I'm 5 feet and 7 inches tall. My face is a 5 to 7. I'm not super hot but I'm not exactly ugly and I have a nice smile. I'm about 130 pounds. Yes, this is still skinny but for my height and frame, I look toned rather than anorexic.
I'm quiet but friendly when approached. I prefer to focus on work rather than socialize. I'm legitimately funny from time to time. I have a few close friends.
In the past, I learned to ignore girls like they ignored me to avoid pain. I still do the same. Recently, it seems like I've been receiving very obvious signs but I'm used to ignoring any and every hope.
Example 1:
I've caught a girl in my class staring at me a few times. When I saw her she didn't look away but I did. I figured she thinks I'm weird or something.
Example 2:
This girl I was assigned partners to for a project was very friendly and laughed a few times at serious things I said. I didn't think much of it and kept working.
Example 3:
I noticed a girl sitting near me glancing in my direction and stroking her hair a lot. I assumed it was for someone somewhere else and left it at that.
Example 4:
I was reclining my feet on my desk and the girl next to me put up her feet so they were touching mine. This caught me off guard and confused me so I said nothing and froze like a dear in headlights. She didn't say anything but she didn't move away either.
Example 5:
There's a girl I sit next to who talks to me a lot and laughs at almost everything I say. Sometimes she sits practically facing me. She's touched me before like on my arm or leg. She's a naturally outgoing and emotional person and I'm the funny guy in that class so I didn't make too much of it.
I don't think I'm terribly hideous but I feel like I'm over analyzing. I used to always make big deals about things that don't mean much. I think most of these girls are out of my league so I assume I'm mistaking these signals for something they're not. So were these girls interested in me or am I being crazy?