i recently made back friends with someone i used to be friends with when i was 11, we have completely different interests, but we get on really well. It might be that if you start talking to these people again you could make friends with them again. The reason people make friends easier when they are young is because they aren't so judge mental and they don't really care what they look like or what others think. In my eyes you have been drained of confidence, which is making it hard to become friends with people, and the sad thing is is that people don't want to make friends with a shy person, but you only get confidence be making friends. For me i had to act really confident to make friends then my confidence grew naturally, it is hard, I'm not saying it is easy, but it is worth a try, if that doesn't work then i would say you were in need of a fresh start, start a new school.