I have been a high school, and community theater, SM for a few years now (I started as a freshmen) and I have a few tips
1) Start off differentiating yourself from the cast. They can't see you as a peer. You have to be held to the level of respect they give the director. Make that clear
2) Talk to the director and have a meeting about what each of your duties are. Also see if the director makes the sign cast contracts, make sure to get a copy of each one, if they exist. Having a signed document showing how each member broke a rule is invaluable in dealing with improper rehearsal etiquette
3) Be on book at all times. You never know when an actor will call 'line'
4) Memorize the rehearsal schedule (or even better- make the schedule). The actors will ask you constantly. Set up a call board as well. Talk to your school administration to find a location
5) Have a master script for all the blocking. This is essential. Especially if the director doesn't record blocking as a few of the directors I have worked for
6) Contact the entire production staff (costumer, tech director, prop master (if there isn't on, find on), etc...) and set up a meeting. You will be the main liaison between these people, so it is important to start off looking professional and on top of it
Private message me if you have any more questions or specific problems
Thanks,
Alec