Lets see he or she is your classmate so it will be easy to find her out.
Option 1: Go to your Facebook home page and and in the search box type the name of your friend for example i enter Tanjil and institution name is North South University. Now you will able to see all the people with the similar name with the institution only.Just add the name of the user and network they are in so you will find them out.Look in the picture below :
Option 2: You can also search people in facebook via their locations places pages they have liked and all other information they have. All you have to do is put name in the search field and after the name put the location or area name and you will have a search results. look below :