I think there is a halfway house.
I am supportive of school punishment like Pitchounette says.
When my ds (7) got a lunchtime detention for not doing his homework I backed up the school and told him why I thought they were perfectly right - but he had already had his punishment, he was upset and sorry and isn't likely to do it again. I didn't think he needed another punishment- I have faith in the school's ability to discipline him.
Mine have never been in trouble for anything more serious than the above homework and for talking in class. If they had been hauled up for something more serious like bullying or vandalism, then I imagine having to tell me about it would be quite punishment enough!!!
Even I draw a line as to how supportive I am of the school, though. When dd's headteacher tried to make her believe that it was very naughty of her to be ill a lot and very inconsiderate to give other people trouble by being disabled, I thought it was more important that dd knew what my real opinion was. If I'd sided with the head at that time, that would have influenced her views of disabled people (and of herself) later in life. I couldn't go that far.
Sometimes important principles are at stake that amount to more than mere protectiveness of your pfb. Imagine if your dc had a racist teacher- would you feel obliged to tell them that the teacher has got to be right? For us it was disablism that meant I could not always be wholly supportive of the school. We are talking about things like making a child crawl on her hands and knees into the toilet to save having to open the disabled toilet- I couldn't possibly have told her that was right, could I?
But I have never made that an excuse not to back the school up on ordinary matters of discipline. And that is what I basically want my dc's to take away and remember: we obey laws even if they seem harsh or a bit silly unless there is a serious clash of conscience.
I was horrified when I found some of the things my disabled dd had allowed the school to do to her because she though it was morally wrong to question a teacher. Also, horrified when I found out that she had not tried to stop the council transport driver from sending an autistic child out of the taxi unsupervised- again, because she's learnt at school that it is naughty to question somebody in authority. I told her that if she'd rather see X dead than be though of as naughty, then she would have to live with that decision.
(having blown off steam, I then went and did something about it myself ).
But don't try to come to me for sympathy when you get hauled up for giggling in class or doing sloppy work- that is not a matter of conscience and all my sympathy will be with the teacher! Even if it happened to be the very same teacher who had recently broken discrimination laws with regard to the same child. You still have to behave! I won't have my children grow up uncivilised and inconsiderate, regardless of who is around them.