I've also definitely seen the rise in children picking and choosing who they want to behave for.
I went to school in the late 70s/early 80s and even then there was a significant number of children who played up for certain teachers but wouldn't dare for others. This isn't new.
Surprisingly it was the strictest teacher, the ones that shouted and the ones that used the belt the most, that had less control of the class.
The ones that were respected by the pupils, rarely used the belt, or even shouted, they just engaged the pupils and made them fear disappointment from the teacher. I thought they had magical powers.
Ds(15) has very perceptively voiced similar about behaviour from early in his school years, it is rarely one or other, it is a combination of pupil behaviour and teacher ability.
Somewhere around P3-4 he had a teacher (she was quite young - compared to me! - maybe mid 20s) and he told me one day instead of walking around their little desks she walked over them. It was near parents evening so out of curiosity I asked her about it.
She said it was one of many tricks her retired teacher mum taught her which works wonders (although her take on it is probably against H&S!), if the class starts to lose concentration, do something different to get their attention, divert them from misbehaving and then get them back to the task. Very clever.