The new DfE, guidance, effective 8th March says:
"In addition, we now also recommend in those schools, that face coverings should be worn in classrooms or during activities unless social distancing can be maintained.
This does not apply in situations where wearing a face covering would impact on the ability to take part in exercise or strenuous activity, for example in PE lessons.
In primary schools, we recommend that face coverings should be worn by staff and
adult visitors in situations where social distancing between adults is not possible (for example, when moving around in corridors and communal areas). Children in
primary school do not need to wear a face covering.
We are taking this additional precautionary measure for a limited time during this
period of high coronavirus (COVID-19) prevalence in the community. These
measures will be in place until Easter. As with all measures, we will keep it under
review and update guidance at that point."
So if you're in Primary you only need to wear a mask if you can't social distance from other adults. In Secondary only if you can't social distance.
I think your head has been over zealous in the application of the guidance and I'd take it back to him/her. I agree you can't teach effectively wearing a mask all day.