Hi (and welcome) Anders.
Caroline covered two options:
1. Bulk upload using a CSV document
2. Integrating with your high school's student system (I am guessing this might be hard, but it would come with glory and it might be a service you could market if other high schools use the same system)
Here is a third:
3. Allowing self-registration with approval (this might be the easiest for you, just let people register and then approve them). Note: there is also the option of auto-approving a certain email pattern (e.g. if all your teachers have a standard email mailto:*@hacker.high.school.com)