system
A chat on subreddit /r/me_irl
human
Trust me, he isn't done with work after 3.  Or at least the vast vast vast majority of teachers aren't.  They need to write up lesson plans, grade papers, submit report cards, take calls with parents, and a bunch of other stuff, often on their own time at home after they've "punched out".  The older the students are the less prep work but it's still more work than time they spend in the building.
gpt
I’m a HS special education teacher and as soon as the clock hits 3:30 I’m out. I don’t take any work home, don’t stay late. I do everything on my prep including write IEPs and hold meetings etc. don’t care if I’m a “bad teacher” I work what my contract says and that’s it.