Are you in the USA? It's different here. No-one really worries if you are eating crisps or not. It's up to you. Not very healthy if you eat too many, of course. Are you overweight?
I don't know about your school progress. People, especially young ones, can be mean if you give them cause - but the problem is that you are letting them be 'mean' and worrying about it. As my mum drilled into me 'sticks and stones may break my bones, but words can never hurt me'. You make your own way and decide whether or not you want to pay attention to these people. I wouldn't give them the honour.