I hate it when people assume I'm not serious about being a vegetarian because of my age. I'm 17, but I've been a vegetarian since I was 5. As soon as I realized that the food I was eating was made from animals I refused to eat it. I went through many nights of sitting at the table for hours because I was told I wasn't allowed to get up until I ate the meat. I went through and am still going through skipping ever single school lunch because my school refuses to provide a vegetarian alternative to meals they serve. I've gone through years of awkward dinners at my friends' houses because they didn't warn their parents I am a vegetarian before they made dinner. I've gone through years of scanning menus for vegetarian options when my family and friends are eating hamburgers. I've gone through years of being told "You're missing out!" "You're going against God!" and "God put animals on the Earth for us to eat." So don't tell me I'm not serious about my choice to be a vegetarian, because even as a 5 year old I was going hungry during the school days and feeling inconvienient eating meals at my friends' houses. I've been a vegetarian for 12 years and trust me, I'm serious about it.
Good for you for sticking to your morals and values! A lot of vegetarians start young and in my experience they seem to be the ones who stick it out. In fact it seems like a lot of people who go vegan or vegetarian at an older age do it for diet or health reasons and do not stick with it because of cultural pressure/norms etc. I can see though how others would accuse a young person of doing it for fad reasons. It's just another way for them to dismiss and ignore something they don't understand.
That said, is there a way for you to bring your own lunch to school? How flexible are your parents in allowing you to make your own meals or contribute to cooking and shopping at home? I know that the Vegetarian Resource Group has a huge section online about children/teen vegetarians and how to meet nutritional needs. You could print this and show it to them and show them that it is not hard or unhealthy. They might be more supportive if they see that many other teens are doing it.
I think it is great that more and more young people are going vegetarian. Afterall, you guys are the future. Good for you!