Most school-going children who fast indulge in major sins.

Answer

Obviously the rozah (fast) of the evil person who indulges in all the kabeerah sins you have mentioned will not be accepted. Such a fast is rejected – struck into his face. Every semblance of thawaab is destroyed by the haraam acts you have mentioned. Their fasting is like a chained dog is denied food and water from morning till night. The effect of the fast is utterly destroyed and they qualify for Jahannum. About such damaged fasts, Rasulullah (sallallahu alayhi wasallam) said: "There are many persons for whom there is nothing in their fasting but hunger." If they believe that the sins are permissible, then they lose their Imaan. In fact, innumerable pupils of secular schools and even adults while believing they are Muslims, have long ago become murtads with their beliefs of kufr.