My kids’ school sends email, SMS, in-app, and phone (configurable by me, except for emergency notifications, which seem to either tree or hit them all).

SMS is almost always the one I see first, but they seem to use it as just one layer in a reliability sandwich.

My experience is that it is about 98% of the time.

Every once in a while, someone will send a text that takes multiple days to get to me, but that's about the only glitch I see.