I had the same issues when I was in high school. I would be tired but once I laid down, wham anxiety. I would stay up all night. It was awful. I would feel completely alone and it didn't seem like I could get control. I would get up and do something and when I felt calmer I would try to sleep just to have it happen all over again. I was too ashamed to tell my parents. I felt so weird. I let it take control of me. Get help from your parents and your doctor. You obviously have faced sleeping issues proactively before with the meletonin, don't give up!!
You are probably under stress from school that you aren't even aware of. Ask your parents to take you to your GP so that you can discuss a sleep medication since melatonin is not working. Have you tried one of the over-the-counter sleep aids or are you trying to do it the natural way? Anxiety can be subconscious and when you lay down, you are no longer occupied with doing other things and so the mind is left to wreak havoc. Not sleeping will eventually affect every aspect of your life so you need to get help for it now before you get to that point. I had very bad insomnia, and I was left feeling like I couldn't do anything not even hold on to the steering wheel of the car. So be proactive and either try an over the counter sleep aid or seek the help of your doctor.