College campuses are not for casual entry.
Every university has a gatekeeper for the purpose of student safety, so if anyone can enter the campus, it would be dangerous for students to be there. Generally speaking, if you explain the situation to the doorman and register your personal information, you can also go in, and the library and study rooms of the school are all accessible, but you have to be disciplined on campus and don't do anything illegal.
The university classroom if several classes together in the big class, can also mix in listening to the class, if it is only 20 or 30 people in a class of small class, if mixed in, it will be very embarrassing, as long as the teacher does not let out of the name, should also be able to, if someone asked to say is the wrong classroom, in the university classroom is not fixed, this is a common thing.
Expanded
Schools are generally not open to tourists, the university is a place of learning, there should be quiet, if open will have an impact on the students' learning early, people come and go to read quiet down. There are also most university campuses are open to the public. Visitors are free to enter the campus to visit the campus environment during the time period when the campus is open.
Of course, there are some schools run by research institutes that require a license. There are also some schools that require swipe cards for campus security, and these schools are not accessible. However, the general public universities are allowed to enter, but it is not recommended to organize a group to go in, the visit is still possible.