Don't live in fear - go to the police station without your child but with your ID and some ID for your child and ASK them if anything has been recorded.
I'll bet you nothing is on record cos the police realised the school stuffed up when they investigated e.g if you have no copy of a behavior management method that should be used, then odds on school didn't possess one!
If the police really thought there was a case to answer from their pov you would have heard about it the same day the incident occurred or very, very shortly afterwards! Education depts frequently flout the law, luckily police depts don't.
To be fair it sounds as if you could probably have fought the exclusion and won. have you taken a look at the IPSEA guidance? (google for their website)
What you can do for the future as a general thing is to get one of those "Autism Alert" cards from the NAS for your child to carry. This has really helped a local Aspie who is six foot seven (in his twenties now)to be seen as the potential victim that that he really is, & helped by police, store security guards etc rather than treated like a violent criminal. It came in so handy when he was learning to catch a bus to the shop with his NT friends, getting lost when he first went clothes shopping by himself etc.
My nine year old has one in case he gets separated from me as busy stations etc, or gets stressed on a couple of mainstream activities he's doing over the summer. They are handy!