Is it possible to go to airborne school if you are in the Reserves?

Yes. I send a half-dozen or so a year.
If you are in a unit in jump status, most likely they will make you go.

It's possible, although you unit has to have slots. Most times they'll send only people with high PT scores or if you win Best Warrior Competitions.

Anything is possible but just depending on situations. Some units will not send SMs to certain schools unless seating in a slot that requires the school. “Just because” you want to might not always happen.