Sara,
Welcome to the forum.
UC is a tough disease,no question about it.
And I am sorry you're feeling so depressed and so down about this horrible disease.
Thankfully you've found us, and we're a great place for you to vent, get advice, etc...
Is there a way you could speak with you guidance counselor at school?
Perhaps he/she could act as a go-between you and your teachers - for purposes of explanation your medical condition and all its repercussions, and maybe giving you extended time to complete assignments. Also for the purposes of "unloading" to someone local, right there on the spot - who can be supportive of you.
What do you think?
I agree with aguywithuc, about diet - have you experimented at all and seen any changes?
Whatever you decide, you should know we'll always be here to support you.