The class is over

This is one of my favourite expressions in English, first of all because I use it a lot at school and when you say it after a hard lesson( so, quite often ) you feel relieved ( oh my god, what a hard work we've got!!). Besides, it's very funny because after a few days at school, children learn it in a natural way and many times, when we are in class and time's up, I look at my watch and say: " Well children, ...... " but I can't finish the sentence because they say altogether loudly: " THE CLASS IS OVER !! "