I always like to hit on a few things; when I'm in academic settings or working with interns, I try not to assume that they're overly familiar with the process.
I like to hit on
Often times, our rehearsal spaces are shared. It never hurts to offer a friendly reminder with a younger cast.
Does your school have an attendance policy? It's worth covering. Often schools with theatre programs will give a participation grade, and being tardy several times can ding it.
I always like to remind people that the schedule can change frequently; even if it's pretty solid, there's still the possibility.
I always like to cover this one; different directors have different notions on what is an and is not allowed in the rehearsal space. If the director is fine with laptops and cell phones and the like, remind them to silence them for rehearsal. Also, communication for you; I typically give out my cell phone number, and I like to note that I'll respond quicker via text, as I am typically in meetings or rehearsals.
This also depends on your school. I like to do this when I'm working with younger casts so that they are familiar with the process should they start working professional and go AEA. Even if the rehearsal won't run according to AEA rules, I like to elect a deputy and a dance captain. It helps ensure the cast can practice the proper way to work in theatre. (Note: I typically select the dance captain with the choreographer, which often doesn't take place until later, but it's worth talking about at the first rehearsal).
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Those are the big points I usually hit on.
I'd also be careful how you phrase 'let me know if you're going to be late', as it may make it sound like it's acceptable. I like to remind them they are expected to be here and warmed up at the start of each rehearsal, and of any relevant attendance policy, but if something should come up, that I should always be their first call.