Please help!! Personal Issue!!?

Im about to be a junior in high school, and am 17 years old. This freshman girl ive known all year is 14 years old and a pretty big girl (not that theres anything wrong with that., and told me she likes me and if i could be her boyfriend. I told her I didnt like her like that, and she preceeded to slap me in the courtyard during lunch and yelled "youre just telling me no because im fat!!" Now, people are treating me different, including my friends, and now even my teachers are treating like im a horrible person. What do I do? How do I convince almost the entire school im not a shallow person? And more importantly... I dont feel like I told her no because of her weight, but did