I don't know if it's illegal or not, but it would probably be best if he wasn't parked there so that emergency vehicles called to the location could park there as quick as possible.
also he would most likely know when it is coming
Only if he had constant access to his police radio. I assume he doesn't since he is parked and away from his car for most of the day, and the dispatchers aren't too likely to contact him directly if he doesn't really respond to emergencies.
Also, if an ambulance is called to the school, then the 911 operators don't really go and call in on police officers in the area do they?
Surely an ambulance or fire engine could be dispatched to the school and he not know of it, and then he would be in the way.