There are several issues here.

Schools are private property. It is up to the school's owner or managing authority whether or not they allow pupils to take photos on their premises. However unless they notify the prohibition to pupils, it's allowed. The school's disciplinary code may have something to say, but there is no legal right to delete the photos unless written into the school's contract with parents. Temporary confiscation of mobile phones is commonplace.

I doubt most schools do prohibit it. Photography is part of the curriculum at many, and corridors are often decorated with pupil photos of events. I frequently took a camera to school in the 1960's, and my first ever publication was of a playground scene, in the school magazine. The headmaster likened it to a Lowry, which was the only nice thing he said to me in 8 years. It would be appalling if such things are no longer possible.

There is also a consideration of privacy, but you would find it difficult to argue that the classroom is a place where you have a reasonable expectation of privacy. In the staffroom or toilets would be different. CCTV is deployed in some schools, which makes the argument even more unlikely.

On the other hand, if the photos are defamatory, humiliating or malicious, or if the photos are uploaded to social networking sites such as Facebook, any defamatory use etc may be actionable. Given the public sensitivity surrounding minors you would probably find it easy to persuade the site owner to take them down rather than face a barrage of negative press. You probably don't want to threaten a libel action against some kid's parents, for much the same reason.