Totally unnecessary drama, largely unwanted side effect of successive governments' attempts to make each local pool of schools compete over admissions.
'Look at us with our shiny blazers! & complete absence of pink hair! We're obviously the best! Not like those scruffy buggers down the road in polo shirts!'
Clamping down on uniform periodically is a quick, useful shorthand by which a HT can convey a message to the kids that they are under scrutiny generally.
Getting the girl with a lip ring to take it out won't make her achieve better GCSEs - in fact, it might piss her off enough to have the opposite effect - but it sends a message, to her peers, from the school, that Yes, Actually We ARE The Boss Of You.
I really don't think it does anything for academic achievement. The only correlation I've ever noticed between appearance & attainment in my subject (English) is that the goths & emos seem to do very well. On that basis, maybe I should give up all that teaching I do in afterschool A/A* booster sessions & just have at the conventional looking ones with the Directions hair dye?
I do wish people would give over with blaming teachers for this sort of wholesale pettiness -I don't know any classroom teachers who give a chuff about kids' hair, earrings or shoes. The rules on student appearance are set by the governors or the senior team, & we're expected to enforce them, which I think most of us do conscientiously.
But it often seems to be implied that we get some sort of satisfaction out of confiscating jewellery & nagging about hair colour. I've no idea why. Arguing with teenagers over stuff they're only going to sulk about isn't fun unless you're another teenager, & why parents of teenagers, who know this, think that it's something their dc's teachers do for a laugh is beyond me...