I saw something about this on Netmums, when I googled it and was also told by someone at the ACE helpline a while back that they can't punish children for breach of uniform policy,
however when I looked at the updated guidance for schools on the DofE website, it says a headteacher can discipline a pupil for breach of uniform policy.
I need to find the bit, if there still is a bit, that says they're not allowed to do this (talking about primary school if it's relevant).
Thankyou anyone who knows about this stuff.