1) Am I required to tell anything to the school? Telling my side of the story would also reveal that I consumed illicit substances, but I am concerned that refusing to talk would implicate me.
No. In fact you are not even required to tell anything to the police if you choose not to either, though that would be inadvisable.
The school has no power over your actions outside of the school, they can't suspend you nor expel you for actions taken outside of school grounds, especially non substantiated ones.
Now I don't know what country you are in, so I don't know your countries age of consent laws, including Romeo and Juliet laws. Frist thing you want to do is look those up to know where you stand legally.
However if it was consensual sex you really don't have much to worry about, as she has to prove you did rape her, you don't need to prove she didn't.
An just so you know, the school CANNOT demand information about your sex life, as they are not a law enforcement agency.
2) What can the school legally do without evidence? The girl's claim is not true, and she does not have evidence, but regardless I am concerned that I will be punished. In the final months of my senior year, I can't have my future jeopardized.
They can do nothing without evidence & if they attempt to do so, you can file a grievance with the department of education in most western countries.
Your exact rights depend on what country you are in, but mostly schools are a toothless clawless tiger, all roar, no threat.
3) Could I use my two friends who were in the same house as witnesses? They can attest to the fact that the girl and I slept in the same bed and that there was no sign of rape.
4) At what point do I need to hire an attorney?
If and when the police get involved. If the police don't get involved there is no point.
5) In the event that this went to court, would I be tried as an adult? Again, I was 17 when this occurred, and I have no past criminal record whatsoever.
6) When should I tell my parents and have them get involved?
7) In general, what should I do in this situation? What do I need to prepare for my probable meeting tomorrow? I have no evidence of anything either, no text messages or anything, just the two other classmates who were in the room next to us who were witnesses.