Ok then, this is not cool. First, talk to someone at school if you can. Second, if you have any choices in your food intake, try to make them be as nutritiously sound as possible - get the vitamins in you and as much protein as possible. Third, if they are only monitoring volume of food and not actually measuring/recording calories, then try to include calorie dense foods like nuts and peanut butter - small amount of those can add up to higher calories (if not allergic). While I don't typically advocate liquid calories, in this case, try to get juice or protein drinks in you.

Finally, like I said in the first place, this is not cool and you need to reach out to someone you trust in your area - school, church, boys/girls club, your doctor (a free clinic if there is one in your area). I don't know your whole story, a medical doctor would be the best one to provide advice as well as encouragement to your parents as to the appropriate way to meet your nutrition needs.

Even if they are recording your calorie intake, you need to make every one of those calories count towards your health until you can get the situation changed.