I thought they were common, though I am old AF, so maybe it doesn't happen as much anymore. Was pretty common from grade school all the way through high school.
Felt like the same for me: every 2-3 months, there was a big assembly in the gym and we got to skip out on class for an hour or two while someone talked at us kids. I don't know how much info I've actually retained from those assemblies. I remember McDonald's burger guy, one time someone brought in a giant harmless snake and we all got to touch it, and one time there was a former pro wrestler who ripped a phone book in half lol.