Feeling out of place

I'm a 17 year old girl in Year 10 secondary school. I feel really out of place and uncomfortable with the three years age gap. I just feel like I don't belong. I also feel like it means that I am developing more slowly. The idea that I won't be leaving secondary school until I'm 21 freaks me out and embarrasses me. I feel as though I won't have the same experiences that normal 17 year old teenagers have until I'm much older due to the fact that I don't have anyone my own age to be with and everyone in my class is obviously at a different, younger stage in their lives. I've had to repeat so many times because of moving around the world and settling into different school systems, it has nothing to do with my capabilities. Despite this I feel that people who don't know this, don't know me ( this includes my own classmates) are constantly judging me. I see the look of pity in their eyes because they think I'm stupid and not able to do stuff which is completely not true, I do well in most of my subjects, yet they still judge. And when I do do well, I feel like I haven't earned it. My classmates glare at me at say it's because you have more experience, you're smarter because you're three years older. It's soul crushing and hard for my self esteem when people think I don't have to work hard fo my good marks. They assume I've done it before and because I'm older, everything is easier and don't work. I'm deeply unhappy in school. I walk in everyday and think about how I don't belong, how I need to get out of this hell called school. People my age should be thinking about universities and stuff. Finally leaving. But I can't. I'm stuck. I feel my classmates are afraid of me. Think that I'm older and don't fit in with them, I'm the outsider and I won't understand what's going on in their stage in life. Like I'm an alien to them that they don't want to accept or take in because I'm different. I do have good friends but even with them everything just feels so wrong. I feel like even they think I'm some sort of freak that isn't or shouldn't be a part of the group. I walk around feeling so alone even when I'm with people. I'm also unhappy in myself, my experience in school is making me hate myself, everything about me. I think I may even have some form of depression. Everyday is a struggle to do the normal school things. It simply drives me crazy that I will be doing this for another 4 years. I need help to sort out my feelings. Understand why I feel like this and why I can't just accept myself and everything the way it is. Why I can't just brush off these awkward, lonely, outsider feelings