If exam boards make a mistake in the questions they set (see http://www.bbc.co.uk/news/education-13978609 for a report about Ofqual’s inquiry) , can a student or parent sue? The private law possibilities might be (1) for breach of duty of care: but is a duty owed? (2) breach of contract, the consideration being the entry fee. But if there is a contract, is it with the school, which will normally pay the fee, or might the student or parent have rights under the Contracts (Rights of Third Parties) Act 1999? Even if a cause of action could be established, the student is likely to face the perennial causation and damages difficulties in private law education claims (“what would have happened if there had been no breach?”) but if anything exacerbated because many other students will be in the same boat.
Exam boards may well be public bodies for the purposes of judicial review, but where a board makes a mistake, it is hard to see what decision is judicially reviewable, apart (conceivably) from a failure to mark fairly in response to the mistake it made – but even there, the Court may say that this is an academic matter, and not one it is going to interfere with.