As a current undergraduate sophomore, assure your granddaughter that, even if it seems like everyone is doing it, be it drinking or hooking up, there a lot of people that don’t. I go to a secular school and the majority of students are not the stereotypical partiers. This is especially true for low income/first generation students who are especially conscious not to lose their scholarships and want nothing more than to move themselves up the socio-economic ladder. This is also true for students with disabilities who may need to work twice as hard as the average student.
Also, it may seem like everyone has the same viewpoint, but it is really a loud minority made up of largely the same type of people. Students like myself who are first generation and studying a difficult field often tend to hold more Catholic values, but do not have time to engage the opposition because we are too busy studying/doing research/doing work-study. I wish I can give a counter-argument to the folks who have the bizarre mental gymnastics we often see today, but I need to give up everything for school.
Here is a story: my friend, also a Catholic, was in a Spanish class here in which they had watched a Spanish movie about a man seeking assisted suicide and they were debating it. She was the only person arguing for the pro-life stance and the professor was giving her a hard time. No one stood up to the professor, but, in the class evaluations at the end of the quarter, most of the students detailed the event and criticized the professor for being trying to block opposing viewpoints, even if they themselves agreed with the professor. The professor in question two years later does not teach the class anymore and, I am not sure whether this episode was the reason why. Regardless, when you think you are alone on campus, you are probably not. Students might be afraid to speak up, but, are there nonetheless.
Also, the biggest hardship I have faced transitioning from 14 years of Catholic schooling to university was not so much being exposed to people of different beliefs, but adjusting to the fact that professors are not really educators, especially in STEM. I have had amazing teachers at my Catholic schools that were teaching clearly because it was a labor of love. So, it was really hard adjusting to the fact that many of my professors either don’t know how to teach and/or are only doing it because they have to and that I mostly have to teach myself the material. This alone made me wish I had gone to a public school. Hopefully your granddaughter is not going to a big research institution like I am in.