"我们家连枪都没有", do not know y u

repeat this many times. If u r sure that is a joke, then teach her a lesson, that is really a bad joke, which can cause a huge trouble. The punishment for a bad joke from a 12 year girl w/o other behavior problems is to kick] her out from the program. If the girl is in high school, the punishment can be even bigger. 

If you do not teach, who do you expect to teach her?