My son too is in 6th form and whilst they do not have to wear uniform smart dress is expected.
Boys, shirt, tie, smart suit-type trousers, black shoes not trainers, no logo-s on clothing, no 'obvious' jewellery only wrist watches or studs in ears only.
Girls, blouses or tops, knee length skirts, bare legs are discouraged as are 'over the top' shoes, no bare bellies or low cut tops, no make-up or 'unusual' hairstyles. Jewellery, as for the boys.
Both are encouraged to wear suits, and many do.
This dress code is 'policed' just as strictly as the uniform for lower years, and boy are they strict.
Nobody objects and they all look really smart going in, in the mornings. We think its a good first step to dressing how many of them will have to for work in the years to come.