Do you automatically defend your child...no matter what?

You need to lay down the law.

A parent's job is to protect their kid, but spoiling them does them no favors later in life, in fact it turns them into selfish little pricks.

Of course. Everyone must let their kids know when they're doing wrong and punish them accordingly. Else they grow up to be just another arrogant f***er roaming the planet today.

Listen to both sides and determine who was in the wrong. Sometimes it's the teacher, sometimes it's the kid, sometimes it's other kids manipulating a situation (a situation my kid has been in before). But if a kid cannot stop disrupting, there might be something else going on. A lack of sleep, ADHD(if the child has always been like this), emotional disturbance, etc.

I believe in getting to the heart of the problem, no matter what that might be and dealing with it at that level. No one get's a pass from me until I determine their innocence.