Pretty much everyone doing work experience in the UK is entitled to the National Minimum Wage. The national minimum wage is £6.15 per hour for people aged 18-20 and £7.70 per hour for people who are 21-24. Those over 25 are entitled to £8.21. However, there are some exceptions that mean some people may have to work for free.
Students who are required to undertake a placement for less than one year (even just one day less), as part of a further education or higher education course are not entitled to the National Minimum Wage. Although many people do get paid while they are working on these prolonged placements, there is no legal obligation for companies to pay.
Another exception applies to students who are of compulsory school age. Essentially, this means people are not entitled to the National Minimum Wage if they are in year 11 or below. Also, if people are volunteering or taking part in any of the following EU programmes: Leonardo da Vinci, Erasmus and Comenius, then they are not entitled to the National Minimum Wage.
There are a few other exceptions too, but as a school leaver it is unlikely they will apply. Find out more, and how to make a complaint if someone has not been paid wages owed to them for work experience, at the HMRC website.