I think being vegan is always a great idea
I went vegan when I turned 13, that was the deal my parents and I made. So they had like 3 months to get used to my nutritional needs etc before I did. Looking back I would have done it sooner - it is a LOT easier than I thought it would be. At first I relied on protein shakes and vitamins for peace of mind, and I still take the vitamins (my family always has, it's not just a vegan thing) but I'm a lot more conscious of getting the things I need from my food - they're just a good backup.
My parents didn't like the idea much, but they got used to it. I just told them how much it upset me the way that animals were treated and, because I knew it wasn't necessary, and I didn't want to participate. Also, they knew that I'm pretty stubborn and there wasn't anything they could do to stop me - I'd be a healthy vegan or an unhealthy vegan, but I wouldn't be a part of animal cruelty. Of course, I said it more nicely than that
With school lunches I just take my own. A salad with spinach, tofu, chickpeas, other veggies and some flaxseed oil and balsamic vinegar takes like 5 minutes to prepare, and it's delicious too. You will get a bit of teasing from other people, but educate yourself and be confident in your decision and they'll leave you alone. Also, people seem to be more open to the idea when you don't talk about it too much - people will ask questions and not care about the answer, they'll just tease you. So be polite about it, think of short answers you can give to people, and tell them to google it if they want to know more, and then change the topic. A couple of my friends and my mum have gone vegetarian, and one friends has gone vegan too