and a couple of friends, made a silly decision, out of school, (went somewhere they shouldn't) which, had they been caught, would have probably resulted in a telling off by the police
It sounds like your son isn't the only one engaging un reckless behaviour, so if these other kids have not also been told they cannot go, and the school can't tell you the exact scenario they feel they would not be able to keep your son and others safe in then yes their decision is discriminatory. Especially because up until that point they were perfectly happy to take your money and felt he was not a risk so offered him a slot on the trip in the first place.
and a couple of friends, made a silly decision, out of school, (went somewhere they shouldn't) which, had they been caught, would have probably resulted in a telling off by the police
It sounds like your son isn't the only one engaging un reckless behaviour, so if these other kids have not also been told they cannot go, and the school can't tell you the exact scenario they feel they would not be able to keep your son and others safe in then yes their decision is discriminatory. Especially because up until that point they were perfectly happy to take your money and felt he was not a risk so offered him a slot on the trip in the first place.