Peoppa,
3 items of clothing are named.
2 relate to girls, 1 relates to boys. Are you saying that, in order not to be sexist, the school should have made it an even 2 and 2 - so a 'no tracksuits, football shirts, strappy tops or short skirts'?
I can understand that it could be seen to be sexist if there were 8things listed that a girl cannot wear, and none for boys, but a difference of 1 in the number of items mentioned relating to boys vs girls doesn't seem to merit a 'so inappropriate and unprofessional' comment?
The school's comments will have related to a) what the church would accept (it is common for female visitors to catholic churches in Europe, IME, to be asked to cover shoulders, so that 'strappy tops' comment may be generated by the church not the school) and b) their own observation of inappropriate clothing worn on similar occasions (tracksuits by boys, short skirts by girls).
Again IME of visiting churches all over the world, both sexes can wear adequate shorts and T-shirts without any issue, but those with bare shoulders and very short skirts / bare midriffs can be asked to cover up.
YOU may never dress your child inappropriately, in which case these guidelines aren't for you and you shouldn't be offended by a general message of no direct relevance to you. However, ask any school teacher in any school that ever has non-uniform occasions, and they will assure you that there are a surprisingly high percentage of entirely normal parents who allow their children to wear the most astonishing things on non-uniform days. If it isn't a public occasion, then we will deal with the issue through lending appropriate clothes, providing e.g. uniform jumpers for playing outside and in extremis ask the child to change into PE kit for the day. If it is a public occasion, especially one where parents will also be present, having someone totally inappropriately dresses is harder to remedy discreetly at the time, hence the guidance beforehand.