The birthday gift for your girlfriend can be chosen according to her preferences. For example, if she likes to listen to music, give her an MP3, and if she likes to read, give her a good book. This depends on the person. You can give her what your classmate likes.
You can also choose gifts according to the personality of your female classmate. For example, if she is a cute girl, give her a plush bear, a cartoon bouquet, a sun jar and other fun gifts. For example, if she is a very fashionable girl, give her some personalized small accessories. This depends on what kind of girl your classmate is.
You can also give her some more practical things, preferably something she can use frequently, so that you can show your attentiveness and care for your friends. For example, if a girl likes to dress up, give her some frequently used cosmetics or some small accessories.