Triwizard or rather, Pentawizard Tornament

Hogwarts will be hosting the legendary Triwizard Tornament, where three students, one from three different selected schools, are chosen to compete for eternal glory. The new rule of having to be 17 or older leaves many furious, but why are five wizards selected, and how are two of them younger then 17?

This is book four in the Severus and Lucy series.
Book one is Snape's Secret: Lucy goes to Hogwarts
Its summary is the following

Professor Severus Snape has a secret and nobody can discover what it is. Somehow it involves Lucy Pevensie, an apparent muggle born witch who has been invited to attend Hogwarts.