The statutes of the USC recognise Galician as the working language of the university; they also stipulate the USC's commitment to contribute to the normalisation and the development of the Galician language. Spanish is also considered an official language.
At the USC Galician is the usual language in the institutional, information and administrative usage: most of forms, application forms, pamphlets, and other types of documents will be supplied in the official language of the university. Also faculty, students and administrative and service staff use to speak in Galician . . . So at USC you are going to hear it in both formal and informal contexts.
About 20% of teaching is done in Galician, with great variations between one school and another: in some, it's up to 60% such as Computer Sciences, whereas in others it is as low as 5% such as Law and Biology... Legislation establishes that students and faculty have the right to use either Spanish or Galician. It's possible that you'll find teachers who teach in Galician, and their choice must be respected. Of course, you have the right to respond in class, write papers, take tests, etc. in the language of your choice, independently of that of your teacher.
As you can see, it's a good idea that you get used to communicating in Galician, or at least being able to understand it, not only because there is a good chance that you could be taught in it and, but also it is used extensively in other institutions and in the media (there are both public radio and tv stations that broadcast only in Galician and other media use it to a certain extent), etc. And, above all, it's a language spoken by many of your new friends, classmates, acquaintances and other people you will meet.