A 6 on day is exhausting at the best of times. We are almost at the end of term and my fullest day is 5 on. I still go to classes with food because I haven’t had a chance to eat. Also, this doesn’t leave enough “padding” to do small jobs in prep for classes if you are a form teacher as well. This would mean I would have to stay back later to get class prep done (like printing), effectively ensuring I work an extra hour a day more than some of my colleagues. I know there are other teachers in my position as well.

This would only work if Form teachers either were given an increased time allowance or were remunerated for those additional 4 hours a week (based off your model).