If teachers didn't inform the school that they were striking and then didn't report for work then it would be unauthorised absence so would be unpaid and disciplinary action could follow.
The Heads had to make a judgment earlier in the week. Some would have felt they couldn't risk leaving the school open if there was a likelihood that staffing levls would be inadequate.
My school was shut but non-striking teachers had to attend or we would have been in breach of contract. We had a full day in, we had to be in on time, wear usual suits, and leave at the normal time (in fact we had an after-school meeting which overran). We used the time to catch up on marking of hundreds of books and change the layout of some classrooms, and we were given projects to do that were not initially due to start until September.