On the surface: white hair represents your sorrow, and this female classmate in my dream (ostensibly a classmate, but actually representing your current girlfriend) has solved this sorrow for you.
Deep meaning: you subconsciously yearned for or once yearned for a girl in that period, and the girl you yearned for has something to do with this female classmate in your dream. Of course, this idea may not be very strong at that time, so now I can only feel it in my dreams.