I had mistakenly accepted a gift from a nabaaligh girl pupil.

Answer

You have to make more effort to locate the girl. If you are unable to locate her or her parents, then keep the gift by you for at least a year in the hope of finding her. After the year give it to a poor Muslim or you may use it and pay its value to a poor person.

However, if after having given it as Sadqah to a poor person, the girl is found then you should explain to her what you had done. Obviously she will accept it. Assuming it was a lost item which you had found, then after giving it as Sadqah if the owner surfaces and demands the item, then you will have to pay for it and you will get the thawaab of the Sadqah.