You throw kid A out of the class for behaviour, but you don't throw kid B out because he is usually OK.

But then kid A's parents get mad - or think they can make some money - and it turns out that kid A is belgian/green skinned/three eyed/an FSM worshipper and you have a lawsuit. Or at least the threat of a lawsuit which can go away with a large enough payment.

Throw out all kids that break any rule and you have been perfectly fair

So the poorest parents in the whole school have a huge incentive to get their kid kicked out of the school for disciplinary problems, especially if they are a minority and can sue for a big settlement. Some people perceive that their most valuable commodity is to be damaged, such that they can sue for it.

In a poorer minority neighborhood near where I used to live I remember reading a report about a super market closing down a few years back -- when times were relatively good. They cited shoplifting and numerous slip and fall lawsuits against them as reasons for shutting.