I have had children go to several different sixth forms, some require suits and ties for boys / matching tailored skirt and jacket for girls. Some have a much more relaxed dress code.
My niece is now attending a sixth form which requires her to wear the matching tailored jacket and skirt, and she bought quite an expensive one, but the skirt is deemed too short and she needs to buy another. These items of clothing cost a lot especially for girls, and she has to wear the same one every day, as she can't really afford two.
My son attended this school more than 10 years ago, and I was happy enough for him to conform to the suit rules then, particularly as boys can quite often pick up suits in second hand shops, so nothing like the expense. It was, and is, a great school, he did very well, and has gone on to a great career.
However, he has never worn a suit since the day he left school, and nor have my other children. It did seem a bit old fashioned ten years ago, but these days, suits seem to me to quite often be a long way from acceptable business wear, and in fact to denote low status in the work place, whereas successful individuals generally wear smart/casual, or even casual/casual.
Looking at my own adult children and their friends, I know successful young people in music, science, finance, event management, energy, engineering - not one ever wears a suit, or even owns one, I know that by the frantic whats apps about 9pm on the evening before an interview recently when a friend of a friend suddenly wondered if he should be attending his interview in a suit and was messaging everyone he knew to see if he could borrow one, and the answer was no, no one possessed one
So IABU to think the insistence on suits in sixth form is outdated and obsolete, and these days denotes low, rather than high status? I think quite a lot of teachers still wear them, and maybe politicians, but these seem to be the only areas of life where they are still quite common