I think it's becasue of this:
"Young people (of compulsory school age including Year 11) cannot be employed in the following circumstances:
where the child is under 13 years of age (except in specified circumstances)
before the close of school hours on any day on which they are required to attend school or after 8.30am.
for more than 2 hours on any day on which they are required to attend school.
before 07.00 or after 19.00 on any day.
for more than 2 hours on any Sunday.
to do any work other than light work, ie work which does not jeopardise a child�s safety, health, development, attendance at school or participation in work experience.
for more than 5 hours (8 if the child is 15 or over) on any Saturday or on weekdays during school holidays.
for more than 25 hours each week (35 if the child is 15 or over) during school holidays.
Anyone wishing to employ a young person must obtain a permit. The form for applying can be obtained from the Local Education Authority or the school�s Student Services. This must be completed by the child and employer and then sent to the Education Welfare Service (address on form). The form has fuller details of the legal requirements, including local byelaws (eg. a young person cannot be employed for more than one hour between 7am and 8.30am on a school day or two hours between end of school and 7pm)."