Is this secondary? In my experience of working in schools, such an early finish is often due to poor behaviour.
They cut the amount of free time at breaks and lunch back to the minimum, so that students have very little time in which to mess about outside of the classroom
You beat me to it. 230 finishes in my area are almost all schools with poor behaviour reputations.
KurriKawari
I used to work somewhere that had a similar arrangement.
We started earlier (8:30/8:40), had shorter breaks and lunch and then on set days we had CPD and meetings. We also had reduced tutorial time as well so had morning and afternoon registration with tutors rather than what lots of schools do which is 20-30mins a day.
Teaching staff are subject to the directed time budget and heads can't direct staff to work outside of those hours. They're well within their rights to refuse to be directed outside of that budget (enough of our planning falls out of that time). That budget includes all teaching time, PPA, meetings, open evenings, parents' evenings, CPD and training etc
You're on a bit of a red herring with the attendance things. The issue with attendance and punctuality is that students who aren't there are missing out on the content being taught and the skills being developed. They then go in late or to next lesson not having a clue what's going on. A change to the school day is different.
It's not an unusual set up for schools to adopt if there is a specific need, but it is unusual to do it 3 weeks in.