The answer to these types of questions is always “it depends”- on the person, the school, the course of study, etc. My husband and I both went to a CUNY school where most of the student body lived off campus. We were married during most of our time there, but there are plenty of frum singles there too. There is no campus life beyond class and the library, no parties, no craziness. You go there for class, and then you go home- to your parents, your spouse, or your frum roommates. We even had some frum professors, which was great for weekday yontifs- one less class to fall behind in! It was a great experience, and we were happy to get a world-class education without having to compromise on any of our principles or values (or our wallets for that matter), and our friends who went (or still go) feel pretty much the same way. In terms of tznius, once you take dorm life out of the equation, you won’t encounter anything you wouldn’t see in the subway; in terms of kefira, you can pick and choose your classes and your major, and even if something does come up (say, having to read new testament in a lit course) you can work it out with the professor- most are very accommodating.
I would say that one who wants to go to a secular college should go somewhere local where they can live at home instead of the dorms, which takes care of 90% of the problem. The other 10% is solved by having a Rav to consult with, and using your sechel.
Getting married young helps too, although that is not always in your control, and I wouldn’t postpone school while you wait for it to happen. I’m glad I didn’t wait to start- now that we are expecting, it’s a relief to be done with college, and had we not started before getting married, we would still be in school when the baby comes, and while it’s totally doable, I hear it’s VERY difficult.
Another tidbit- I don’t know how it is at other places, but I found that a lot of people in my classes were a little older than the typical undergrad- say 23-25 instead of 18-21. They had gotten some work and world experience before coming to school, and that made for a much more mature group- they were there to learn and succeed in their studies, not to party; when you find yourself with such a group of people, it makes for a richer and spiritually safer educational experience.
Lastly, since you are there to study and get a degree, and not for the narishkeit that happens at many campuses, you can take heavier course loads, thus shortening your stay there. Both of us finished in 3 years, rather than the usual 4. This is a great option for married people (see above), as well as for those who truly view college as a b’dieved or necessary evil.