I'm not trying to rip you apart, Ukulele, but I don't agree with your OP.... you brought uniform when you didn't have to (your bad, but I think it'll work out fine for you because those clothes will get trashed and your child's actual clothes will remain fine, so you forked out when you didn't have to,but heigh ho - put it down to experience and watch those clothes get covered in glue, glitter and paint!). And you object to how the teacher is dressed. I think how the teacher dresses is literally nothing to do with you, but your post(s) are as if it's your problem to solve.
So, hypothetically, imagine you are someone about to be evicted from your home.... you are facing homelessness. So you go to your local council for advice. The person on the desk who you talk to about your issue is wearing a t-shirt which carries the slogan "charity begins at home". You take offence at this, as you are potentially homeless. Is her slogan'd t-shirt more or less important than the advice she gives you? Do you really put in a complaint (or comment to her line manager, who you know personally) rather than listen to what she's telling you and understanding if it's good advice, and you're going to be able to put a roof over your head, or crap advice and you're going to be on a park bench. And if those are your choices, why are you even noticing her t-shirt. It might be policy that staff shouldn't wear t-shirts with slogans. Pointing this out to her manager stops you sleeping on that bench or keeping your roof overhead exactly how?
This person (in your OP) is going to be v important in your child's life until next July.
I'm gently suggesting that your OP isn't the most important thing between now and when schools break up, and (in any case) not your business to point our or correct.
And supposing you do raise this. And then, in 2 weeks time, your child comes home crying. He is saying his teacher doesn't like him. Gives favouritism to others, is mean to him. You've already complained about how she dresses. How do you go into school with these issues if you're already seen as someone who's down on her because she wears shorts when it's hot? I think you should save your complaining for when you've got something really proper to raise. Unless you're comfortable with block-booking the manager's diary out for every Monday morning with your list of complaints about what happened last week.....