Ronnie do you have a statement detailing what the other sides version of events or just your sons side of events......whilst you may not wish to believe its possible,maybe there's more too this than your aware of.
Does sound very odd (from previous thread) that a gang of 20 kids would attack your boy unprovoked...and throwing water.

sorry to say this mate but it doesn't sound as straight forward as it might appear,really think you should (if you haven't already) get a detailed statement from all involved.
I was nearly expelled from school when i was 14 due to savagely attacking a female teacher.....nearly blinded her and of course this was a premeditated attack (headmasters words).....only when the police was called in and parents called out of work to attend school before the police took me away did the truth actually come out,the female teacher simply run over to where i was hitting a class mate (boys will be boys) and accidentally walked into my elbow,she said it was a simple accident and partly her fault.....still classed as a thug by the teachers from that day on.
So you see how situations can be black and white until you dig a little deeper and things become not so straight forward.....i do hope you end up resolving this,just remember as children we don't always tell are parents the whole truth.
