Without a pre arranged ride at a given time you're going to have to negotiate with a driver when you ready to leave
Cap
Linston's Zion Hill Taxi
Captain Dave
What else do you plan to do in Sav? Keep in mind the school may not "want" you later than the arranged time, HOWEVER, I do suggest you spend additional time in Sav. I'd recommend speaking with someone at Rockhouse, given that Sav is kind of a large town... what do you want to do and see.... it may be helpful to have a local driver/guide to take you around to notable or interesting spots. Just some thoughts
I'm gonna be Iron like a Lion in Zion