For a young person who is of school age there are rules and regulations on how many hours you can work. I cant remember the exact rules, (but can find out if you want) however, I know that a young person who is still of school age is not legally able to work during school hours (even if they, for what ever reason, do not attend school).
With regards to an NI number..if a young person does not have a NI number then they are able to get letter from their school or local education authority to allow them to work, this should be adequate.
Pushy11 - if you are asking this for yourself may I suggest that when you go back to school you speak to your Connexions Personal Adviser - they will be able to help you.