There is no law or even a rule that heads must follow - unions would like a certain amount of notice to be given, but heads are under no obligation to follow their 'recommendation'. Not a good idea, was it? The head has every right to ensure that proper, planned lessons are being taught right up to the end of term. I know some teachers like to faff about at the end of term, but it's merely selling the children short. NB was this just before Easter or (as I suspect) just before the end of the half term? If the latter, I'm afraid that's an even worse waste of teaching and learning time.