From my perspective as a former secondary teacher, it's parents. At the least sign of their kids being in trouble (or failing), many parents will come up to the school (that they often never visit otherwise) and proclaim that the teacher is wrong and shouldn't be punishing little Johnny. And schools give in because they're worried about lawsuits and having to deal with parents, so the teachers get thrown under the bus and
can't punish the kids.
Thankfully the last school I worked at did back the teachers - at least to some extent - but it was so annoying having to deal with parents who never showed up to parent-teacher conferences, never contacted me (or answered my emails/calls!) come up as soon as their kids got sent to in-school suspension for misbehaving or for having too many missing assignments or getting a bad grade.
There's also an anti-punishment trend prevalent in education research. Which I truly get, but sometimes kids need to know the limits and you have to do something with the bad ones so the others can actually learn. It's a lose-lose situation (and part of the reason why I left, though I really do miss it and am considering returning, though maybe at international schools).