You really don't have a weight problem. I suppose these bullies believe they are perfect in every way. Well, obviously they are not, bullies bully because they have problems of their own, and bully to detract from their own failings. Have a word with a teacher, I'm sure they will be able to sort this problem, without letting on you've confided in them.
Is there anyone at home you can chat to? A parent, grandparent, auntie, older sibling? Or any teachers or TA’s at school? You sound like you are a perfectly healthy weight but the name calling at school is a real issue. You need some real life support sweetheart.