OP
You are saying your mate has already called the police. Therefore, what can you do now? IMO be supportive of your friend stick together and get others involved. If the parent/s that were assaulted put their mind to it, they could stop the parents from coming to school.
Taking everything at face value I hope other parents unite with the parents who were attacked and send the spitters to Coventry
I've seen bullies in action, f/ing and blinding at people, spitting at people and spitting, hitting, spitting at police until more police arrive on the scene and get taken down to the ground, they soon change their tune EG, "oh you are hurting me...cuffs are hurting me... I did nothing...why are you doing this..etc etc " and their mate/s who were part of it pretend to be good mates and apologising to police ad asking them to let their friend go. So lets get the police and CPS to send them to court, name and shame them in the hope it never happens again/
You state you are worried that things may get worse - they can but nasty, disgusting parasites like the spitters bank on that as they are bullies, until banged to rights
Do keep us updated, please.