What should we do with the money and phone cards left by an unknown girl with our daughter on exam day, which she hasn't returned to claim?

Deposits whose owners are unknown are lost funds. They should be placed in the Muslim treasury (Bayt al-Mal) if one exists and is well-regulated. Otherwise, they should be spent on the disbursements of the Bayt al-Mal, such as public interests and providing sufficiently for the poor and needy. If the owner cannot be reached, the funds should be spent on a public interest, given to the poor and indigent, or sold and their proceeds given as charity to them. As for the issue of whether the daughter is truthful or not, the default assumption for a Muslim is integrity, and ill-suspicion is not permissible except towards one whose outward appearance suggests evil. This depends on your previous experience with your daughter.