In college, I have seen many things. I have seen people go to class while wearing a tiger onesie, professors spend the first five minutes of class trying to throw a marker into a trashcan from across the room, and a jar of peanut butter lying around inside a newspaper bin. I have also learned a lot about social behavior and the rules that keep a culture stable. For example, there is one unspoken rule that it is taboo to break: in the first week of school, you must choose the seat you want to be stuck with for the rest of the semester.
Obviously, there are no assigned seats in a college classroom. Professors try to make it clear that all of us students are adults and that we are not going to be treated like children. One of my professors even said that they do not like when college students sit in a circle because we are no longer in preschool. With no assigned seats, students can sit wherever they like. Want to see everything that the professor writes on the whiteboard? Sit in the front. Hate answering questions in class? Sit way in the back. Feeling a bit like the protagonist in an anime? Sit towards the back, next to a window.
One of the benefits to showing up to a class early is getting to pick your seat without any social pressure. If you show up late to class, you have to navigate your way through the almost full classroom and find an empty desk that is hopefully not being saved by someone’s friend. However, this is not really a problem for those who do not overthink social situations, since they can usually sit anywhere and not feel uncomfortable about it. They just want to get through class.
On the first day of class, there is very little difference between freshmen and seniors in college-- except when it comes to seating arrangements. Seniors likely know one or two others in the class, and will feel comfortable sitting with them since they know those people are cool. Freshmen hardly know anyone and have to gamble that the person they sit next to is not annoying or distracting. Seniors also know that wherever a person sits in that first week of school becomes their seat until the last day of class. Freshmen do not always know this rule and might bring chaos to this order.
The reason I say “first week” rather than “first day” is because in that first day of class, students may figure out where they want to sit, but they don't always stay there. They might realize that the seat they chose is under the air conditioner, or their seat is farther from everyone else, or their best bud is in the class and they want to sit together. That is why people tend to experiment with seats in the first week in an attempt to find their comfort zone. The only time this rule does not apply is in classrooms where the previous class moved all the chairs around. In those cases, someone either has to rearrange the seats to restore order, or everything just falls apart into anarchy.
So, if you are in the middle of your semester and someone stole your spot in the classroom, just casually walk past them and sit somewhere close by. They’ll get the message.