Welp, I was a bit like you at 15, I had friends but they never invited me out, never really talked to me or anything, I just tagged along when we went out and hung around with them. I figured out how to fix that and that was: Get new friends, I helped a guy in my maths class out with homework and then asked to go up with him to lunch, I then made friends with him and all the people in his group reasonably quickly. (during this time I was depressed also, (self diagnosed ovc) as I didnt want to go to the doctors, I could just tell it was depression though due to suicidal thoughts, sadness, crying and the thought of harming myself in numerous ways)
As for the physically hit thing, that happened to me too for a while until I beat the living daylights out of the people who did it, yes I got in trouble for it even though they were the ones who were starting it in a way yet it never went any further than a slap on the wrists by my school as there were fights there almost daily.
You can get friends easily if you talk to people. I moved school recently (not due to bullying or anything, just due to moving house.) and I've got more friends here then I did at my old school, my suggestion is: if the people try to harm you, harm them back just in a more painful way, trust me, they'll stop. Talk to people you may not normally talk to, they'll talk back and will probably treat you better than your other friends.
Also, as for the depression: listen to "Kid Cudi", all of his "Man on the moon II" stuff is pure gold. as well as a lot of his "indicud" album. Got me out of depression as I listened to it on repeat every day. (He's especially good if you like hip hop rap as you'll like the sound of the music also.)
https://www.youtube.com/watch?v=p8EH1_jZBl4 Trapped in my mind - Kid Cudi
to get you started, also consider going to a doctor for you mental health issues too, I didn't do that myself, but it will defo help.