The college of a few generations ago was focused on a curriculum that involved philosophies that were incompatible with the life of a Torah Jew. It was an exception that participating in such programs was without damage. This trend died out, and the college environment was more focused on the specific educational programs that the student attended. However, there was a different risk that became quite strong. The lifestyles of college students involved relationships that are illicit for us, and tznius dipped to new lows. The environment had its toxicities. Those who darshened about college wasted their time, because they addressed dangers that no longer existed, and bochurim and girls heard these empty speeches about obsolete issues. The darshanim failed to be up to date and focus on those issues that were the real risks.
There were several large, mainstream yeshivos that allowed bochurim to go to college, and these yeshivos required specific approval of which college and specific to the bochur. Often permission was not granted. The bochur needed to maintain regular sedorim by day, and only allowed to attend night classes. This had the added advantage of lowering the risk of sharing classes with the riff raff that contained the bulk of the non-tznius dress and activity. At least the philosophy matter was recognized as obsolete.
Today’s college campuses have reverted back to pushing ideologies on the students. The average campus is a cesspool of activism for political correctness, gender fluidity, absence of tznius, and legitimizing things that initially sound praiseworthy but are actually decrepit. Fortunately, there are a few college environments that are safer for our young people, but just a few. Some of the higher levels of academia, where only highly motivated students with a track record of performance make it in, are actually a bit lower risk.
I am not afraid of apikorsus in college today. But the sheer level of stupidity in completely corrupt value systems is alarming, and this is coupled with the tznius issue. One needs to know the risks and how they would affect a specific bochur or girl before giving the approval to attend college today. There is no general answer, not a heter, not an issur.