Swimming is not the issue. This is gym class.
Leg suits ARE available for swimming (mainly for guys with hairy legs who dont want to shave their legs).
Public schools are famous for arbitrary rules. While I don't the theology behind the request here, I do support the rationale of the parent to question WHY a student MUST wear shorts if other safe/ functional attire can be brought to class.
Speaking out of the other side of my mouth, these people bug me. Wait til they tell me Christmas offends them.