As you do. Fandom High applications are open until the 18th, for teachers and students. Characters pimp out the school in the Fandom High Brochure.
And if you'd like an idea of some (but by no means all) of the characters who aren't there but should be, some Fandom High wishlists:
1 | 2 | 3 | 4. Yes, some of those are from last term - most of them are still applicable!
Also, potential characters can (but aren't required to) be tried out in a no-strings, never-happened FH setting at fh_launchpad.
ETA: And not an admin, but happy to answer questions like "Does this game eat your brain?" (yes) and "Did Hamlet, Prince of Denmark really set off a volcano at Prom?" (also yes) and "What's with all the glitter?" (It attracts boykissy).