School is legal I'm afraid. Even if what you say about the human rights act is right, domestic law takes priority over the human rights act. As it is very clearly written that not going to school is illegal in many statutes, the judges cannot "linguistically strain" the intepretation of that statute to be compatible with the European convention of human rights. So all they can do is make a declaration of incompatibility.

There is a convention that any such declaration will require a rewriting of the law. However, conventions are enforced only by public opinion / outrage and I seriously doubt there will be much public outrage against a politician saying, "incompatible with HRA or not, compulsary education stands".

But it is an intesting discussion about wither education is legally slavery.