Is it permissible for a committed girl to talk and joke with her non-committed female classmates, or is it obligatory for her to avoid them, especially since she finds no one else to associate with during a long school day?

One must be keen on righteous companionship. Speaking and joking with non-observant girls is not forbidden, as long as there is no fear of their negative influence on one's religion and morals. One's relationship with them can even be considered an act of inviting to Allah. The saying that one must forbid evil or distance oneself from its perpetrators applies to the presence of evil itself, as in His (Allah's) saying: "And He has already revealed to you in the Book that when you hear the verses of Allah being denied and ridiculed, do not sit with them until they enter into another conversation. Indeed, in that case, you would be like them." The verse does not mean that everyone who commits an evil act must be forsaken and boycotted.