Do you still think this officer was right?
Yes. That boy didn't try to hurt the teacher. If he'd been agressive (headbutting for example), he could have done real damage and, in that situation, she'd have needed to get him on the floor quickly and safely. The teacher also wasn't surrounded by groups of unpredictable people in a possibly volatile situation. I don't think the two situations are comparable.
It is not about being stronger or bigger it is sbout knowing where your body is strong and there's is weak and how to use that information in a crisis in order to get them safely into a position where you have physical control.
Which is exactly what he did. He didn't have 2-4 back up officers as required according to your first link. He was a lone officer is a potentially dangerous situation trying to arrest a girl who was trying to kick him and had attacked another pupil. She was known to him for he violence.
I have an SIA licence (15 years old - even valid?) but as part of the training, we were taught how to get people on the ground quickly where they can't harm you or themselves. I was shown how to get a guy with 1' and 60kg+ advantage over me. Not exactly body slams but similar and with a similar 'risk' to the aggressor.
I think that SIA tactics (for want of a better word) a more comparable to the situation faced by the policeman than the teacher in your video.