Congrats on moving to the next level. I think, overall, you will find HS baseball easier than rec or travel. For the most part, the players do more of what you expect them to do and you'll have less funky plays to officiate.
I'm sure your training will cover HS rules. Some of the major differences:
ALL obstruction is a delayed dead ball, and ALWAYS results in at least a one base award from the runner's position at the time of the obstruction.
There is no "in the act of fielding" in HS. The fielder either has possession, or he doesn't.
Force Play Slide Rule - it's different from the bonafide slide rule in OBR
Runner's Lane Interference - pay close attention on how your association wants it to be called.
Follow-through interference (sometimes called backswing interference) - same as batter's interference. Someone is going to be out.
Pitching positions - I'm not up-to-date on them, but they'll be different from OBR
Dugout Restrictions - nothing similar in OBR. Part of your "toolbox" to manage a game, which will be important in those freshman games that might only have one babysitter coach.
You'll likely be asked to confirm game details a few days in advance via email with the coach or AD. Game date, time, location (JV field, varsity field), and if you'll have access to a locker room (more for varsity). Get examples of emails from a varsity umpire you trust. We always asked our umpires at all levels to arrive no less than 45 minutes before game time, wear decent clothes (business casual) and then go to the field and find the coach to let them know you have arrived and to reconfirm game time and when you'll come back for the plate meeting. There have been several times in my career that the visiting team will be late because of bus issues or traffic. Good to know before you show up 10 minutes before the game starts for the plate meeting and only the home team is there. Your association may do things differently.