Educational Decree Number Twenty-four states that all student organisations, societies, teams, groups and clubs of three students or more are to be disbanded. Permission to re-form can be granted by the Hogwarts High Inquisitor, Dolores Umbridge. Failure to comply with the Educational Decree will result in expulsion.
Cornelius Fudge and Professor Umbridge attempt to use this Educational Decree to expel Harry for the organisation of a secret society. They are unsuccessful as, although they know of the inaugural meeting in the Hog's Head, they cannot prove that any meetings took place after the implementation of the Decree two days later.