I am really skinny?! ?

I'm really skinny but am very fit and eat alot I can run five kms and eat 5 pieces of toast for breakfast every morning I have big lunches and large dinners I once even ate a whole pizza to my self but I'm still stick thin !! I get teased at school as people say I'm anorexic but I'm not :( why am I so thin and how can I gain more muscle to be bigger looking ?