How can I gain dance experience as a high schooler?

Do NOT look on YouTube and forget the dance team. YouTube will not give you the corrections you need and performing dance moves in an incorrect way could result in injury. School dance teams are just for fun, they don't actually teach technique. If you want to be a dancer once you get out of high school you need to find good classes at a professional studio. If you would like to start ballet, you're starting really late. The ballerina that started the oldest that I've ever heard of was Misty Copeland (ABT soloist, started at age 13). I started ballet when I was your age and I realize that it's not likely I'll ever be professional, although I do try really hard and have actually caught up to most of the other girls my age. However, if you want to do any other type of dance, you should take 2-3 classes of that type of dance per week plus ballet 1-2 times a week because it's the foundation of all dance and will help you with technique used in other dance styles. Good luck and have fun!