I am currently 14 years old and am studying Chinese at school. I really like the language and the culture and want to go to a Chinese highschool for a year (or semester). A friend of mine did something similar in France last year and had a great time. I know that China is very different, but I want to do this. Does anyone know an organization that organizes one or two semesters at a Chinese highschool, also with accommodation, support service and so forth?

Hi Susan,
Where I live in China, there are currently a couple of high school exchange students in the local high school. They are studying for a year, taking Mandarin classes (they came as beginners) and several classes taught mostly in English–tai chi, Chinese history, calligraphy, etc. I can try and get you the name of the program they came from, though they are both European so I'm not sure it's available to Americans. Send me a private message and I'll let you know what I find out.

This company seems to organise highschool programs in China http://www.livethelanguage.cn/highschool-china/
All depends on whether you want to attend a highschool with Chinese kids, like a semester abroad in China, similar to what your friend did in France probably or study Chinese together with foreign kids.
Quite an adventure you have ahead of you there.