Other mistake: When Sam is texting Austin, she gets the reply the split second after she sends her text. There isn't enough time for Austin to read the text and reply.
Other mistake: During the pep rally scene, when Shelby is staging the play, Gabriella comes out wearing a bridal gown. A couple of seconds later, she comes out wearing the "diner girl" outfit, complete with roller skates. There was not enough time for her to change so quickly.
Other mistake: When Conner is setting the time for Sam to be back at the diner by midnight, the phone isn't on. (00:32:47)
Answer: Yes, this happens in most schools. The reason is that students are more likely to pay attention to announcements that are not made by a boring school employee. In many American schools, we have a phenomenon known as "home room". This is a short period usually first thing in the morning that is non-instructional when announcements are made, forms are filled out, and administrivial functions that don't have anything to do with any one class. At schools that don't have this, they have a thing called "study hall" or "independent study" which is a period where you sit in a classroom and work on your assignments, special functions for the school admin. like you are talking about are frequently done at these periods by students who are "working in the office" rather than studying. Then, there are schools that actually have classes in journalism, broadcasting, etc. where this activity would be part of the schoolwork.