I deliberately waited this year as to be honest we werent 100% sure what he would be doing given his total lack of work in his GCSE's but in other years I have just forgotten until the last minute!
If your child is staying in full time education, doing an approved course etc then you need to opt BACK IN to be able to continue with your claim rather than cancel the claim if they are not carrying on study.
www.gov.uk/child-benefit-16-19
HTH somebody as forgetful as me!