why don't you try to reason with your kid when he misbehaves? it is obvious he is afraid of his spankings or he would not have told the AP. which tells me he probably gets spanked far too often and it isn't once in a great while. he is old enough that you can reason with him instead of spanking him.
as for school they did the right thing. just like taking him to a doctor and they suspect he is being abused they have to call the law.
if you reasoned with your child instead of using violence you wouldn't have the problem you have now.