5th period? Is this a common thing?

I did my training placement at a high school in Australia a few years ago as Ed Support staff.

One morning I came in, started making my coffee in the staffroom, and went to the fridge to get some milk. When I opened it, I found it was fully stocked with alcohol. Beers, whisky, bourbon, vodka, wine, mixers, the lot.

I asked a staff member about it and they replied super casually, “Yeah, that’s 5th period. On Friday, after all the students go home, the teachers have a little get-together.”

I was honestly kind of shocked and didn't really know what to say. If it's genuinely after school hours and the students have gone home, I guess there's nothing particularly strange about having a staff drink. I'd just never encountered anything like it at a school before.

For clarity, I have no reason to believe the teachers were drinking during work hours. I never saw anyone drinking during the school day and no one ever acted drunk.

There were a couple of things that struck me as odd, though. The staffroom was on the ground floor, there didn't appear to be any cameras covering the area, and the fridge had a latch for a padlock but wasn't locked.

If the staffroom had been left unattended, any student could have walked in, opened the fridge, grabbed a beer and walked out without anyone noticing.

Is this actually a common thing in Australian schools? Do other schools have a Friday afternoon staff drinks culture like this, or was this particularly unusual?