I was diagnosed with UC at 15 1/2 (October 2006, 10th grade). I had been out of school 9 weeks so obviously my friends knew I was sick. My best friends asked if I was okay because they hadn't seen me in a while. I told them that I had UC. I didn't go into detail much. I said basically I have ulcers in my colon or large intestine (I realized people don't always understand they are the same organ). If they ask what some of the symptoms are I say a lot of cramping and trips to the bathroom. My close friends went online and read what it was. My friends are really supportive and when I was sick again this year (October again and now) they would bring home some of the work I was missing in my classes. They always IM me if they haven't talked to me in a few days to see how I am. My friends also like to joke around with my UC. One of my guy friends said he was going to buy me sudoku toilet paper because I was in the bathroom all the time and I love playing sudoku (he never did buy it for me).
It definitely is hard especially at a young age to have UC. If I was 13 and one of my friends told me that they had UC I'm not sure I would understand what was going on. I don't think many people at any age understand what we go through. I would tell your friends that you have UC and if they want to know more than to ask you. You don't have to get very graphic. Sometimes I tell people that I am friends with but not close friends with that it is like having a never ending stomach virus. It usually gets the point across.
Finding a teacher or someone to talk to is nice too. I am close with my 6th grade history teacher. My friend and I would come back to the middle school sometimes to hang out or helping clean his classroom. Not long after I was diagnosed my friend and I were helping him around his classroom. He had known I was sick and I told him I had UC. Next thing I find out is he has Crohn's. He was really helpful when I was trying different medications to find one that worked. He even said it is nice to know someone else who has similar issues. I also found out that a girl in chorus with me has Crohn's. I have never talked to her face to face but talk to her online. She was having trouble also and it is good to know there is someone there for moral support.
Only tell your friends what you are comfortable telling. A true friend will try to understand the best they can. I don't think my friends totally understood at first but they try. And there is always this board where all your friends understand what your going through.