What is the ruling regarding Dhuhr prayer for mature boys in French schools, who are prevented from praying and are feared to be at risk if they leave, and is it permissible for them to combine prayers?

If the student is able to go out to pray in the street without harm, then he must do so. However, if he is unable to pray at all, then he has a dispensation to combine the prayers by delaying them (Jama' Takheer). Some scholars have permitted combining prayers for any general need.