Kids of 14/15 naturally have a rebellious phase and actively seek out ways of kicking back against adults. I loathe school uniform, but I think at that age a school uniform Gives the pupils something minor to kick against. Better that than seeking out other, more extreme ways of shocking adults.
I got the impression, that Mr Drew has seen it all before. The rebellion unfortunately kicks in at 14/15, then disappears after GCSE time. He sees his job as keeping them in school and getting their basic education, as the pupils cannot see how impt it is. For some of the more challenging pupils, by the time they hit 17 and look back on a failed education, they might lack the drive and courage to admit their mistake and start all over again. Mr Drew ensures that they look back and say, 'well I was a little shit, but at least the school stuck by me and made me getbthe basics'.
As for teachers telling them off then being 'chummy' in the next minute, I think it falls into the remit of dealing with bad behaviour, then moving on. There's no need to dwell on it.
I'm surprised the school allowed the 'behind the scenes' footage of the teachers to be broadcast though. I'm not criticising it at all; I just don't think it's appropriate for pupils to be seeing it.