Ah man that is some bad bit of luck there! why have you not talked to him if you want know what happened. Or the police, they were the ones who made the decision of how they chose to react. Anything els is just guessing and speculation.
A boy at my school his friend threw a rock at him and he tried to throw it back but missed and hit a police officer car. Will the police do anything? Or will he get expelled or something? Can somebody please tell me what might happen?
Nothing if you keep your mouth shut.
If the police think he deliberately threw a stone at their vehicle it would be up to them. If he doesn't have a history of temper tantrums there probably won't be much trouble.
If I was a cop I wouldn't be very happy about some jerk throwing things at me and saying, "I didn't mean it, officer. I was really throwing it at somebody else."
Stay out of it. Its not your business.