I feel really depressed?

I'm a 16 year old girl who will be a junior. For this past year, I've felt really down about life in general. My self-esteem has plummeted. Socially, I'm going through these strange friendships that can go either way. And now, we have one day left of school and if I don't do well on my finals tomorrow (English + geometry), I'll have to go to summer school. And particularly this summer, I had a bunch of things planned out to do like going to conventions & going to Oregon for a few weeks. But that can't happen if I have to go to summer school. :/ I've never failed a class in my life, but now I'm not too sure. Now I feel like there's no point in finishing high school because I won't get into a good college. I think my recent lack of motivation and bad grades are on part of my dull outlook on life right now. What can I do?