We're wearing mufti because it's a risk to wear normal uniform since many people only own one blazer, tie, etc. But we're not allowed to have our shoulders or stomachs out which is very restricting considering almost all my clothes (and almost all the items on every woman's clothing site ever) breach this rule..
I know that the government have put a ban on borrowing equipment to minimise the spread of the virus, and I doubt the school will be allowed to give out detentions. I suspect you would either be asked to leave school, go home and change, or maybe given behaviour points (or whatever they're called at your school)