I sat next to a girl in class today who said she had the stomach flu, will i get it?

Stomach flu: Stomach flu (gastroenteritis) is generally shared by poor hand hygiene. If you are not washing before eating or touching your face and you and she were sharing objects especially food or drink then you will get it. If you were just sitting next to her it is unlikely. Here's a bit more information on stomach flu: http://www.Cdc.Gov/ncidod/dvrd/revb/gastro/faq.Htm.