sm is correct, legislation is different for teachers/school staff.
I am a teacher, and train other teachers/support staff in positive handling (restraint). I have taught in mainstream, SN and EBD settings.
The legislation needs to be clarified. Presently, a teacher/support staff can use physical force if:
-A child is injuring others
-A child is injuring themselves
-A child is damaging property
-A child is behaving in a way that is likely to disrupt good order.
Any physical intervention taken must be reasonable, proportionate and necessary.
This means that if a pupil is charging at another holding a knife, you are justified in rugby tackling them to the ground- if they are verbally abusing you, you are not.
The vague definitions do not help- 'damaging property' could mean smashing a window; it could also mean snapping a pencil in two. Legally, you could restrain a child for this, but ethically? It would be completely inappropriate. The legislation relies on teachers' judgements of what is reasonable, proportionate and necessary, and there is rarely any provision to train them to effectively make that kind of decision.
As a teacher, you have a duty of care to keep children safe, and act in loco parentis.
So, even if, as it appeared to you, the teacher was indeed "using physical force to get a child to comply with instruction", they would still be acting within the law.