Yes, normal in other counties as well. We are in Frederick and my DS is at home today because it's graduation day for his school.

yup.

It's not "unofficial" it's just a different day for each high school, but that specific high school is officially off that day.

Yes.
It caught me off guard last year but it makes sense.

Yes, normal. The teachers and administrators are attending graduation with the seniors.

Yes. Who would run the school with the administration and teachers away at graduation?

My son is a 10th grader. His school is unofficially closed on graduation day next week. Students are told that there’s no school, but on the calendar it’s an instructional day. Is this the norm in high schools in Maryland?