There would be a lot more trips to Hogsmeade.
For butterbeer, of course. If there’s one thing college students like to do, it’s drink. Because the drinking age is 18 in England, college witches and wizards of all ages would be able to go enjoy a pint at The Three Broomsticks instead of bribing their older friends to sneak them a butterbeer or two.
The Great Hall would have to be open 24/7.
Opening just for breakfast, lunch, and dinner would not suffice. College kids eat at odd hours, especially after a night of drinking, so there needs to be some food options available. There’s probably not a Taco Bell nearby, so the Great Hall would have to stay open and serve a late-night menu filled with carb-loaded, greasy deliciousness.
Dementors would suck the life out of parties.
Instead of the cops coming to break up the parties, the dementors would take on that role at Hogwarts College. Anytime the parties started to get out of control, they’d just have to show up and everyone would scatter like flies. Losing your soul is a much bigger deal than getting a noise violation.
People would care more about Quidditch Beer Pong than actual Quidditch.
Sports are fun, but sports that involve copious amounts of alcohol are even better. Everyone would fill up their red solo cups with butterbeer and try to throw the ping pong balls through a series of hoops. Just like in regular beer pong, the more you drink, the better you get.
Everyone’s Gringotts vault would be empty.
True to the college experience, everyone is broke. The only exception is those fortunate few whose parents still gave them an allowance. The average student at Hogwarts would have to resort to the cheapest foods and drinks that only a few Knuts can buy. At least these guys don’t have to pay bills, so they don’t have to choose between hot water and dinner.
People would go to the Forbidden Forest all the time.
There was a nature reserve right by my dorm room in college, and everyone called it the “enchanted forest” because they went out there to smoke and who knows what else. You know that if Hogwarts students wanted to use illicit spells and potions, the Forbidden Forest would be the first place they'd go.
Time-turners would be highly coveted items around exam time.
A time-turner is a procrastinator’s dream. Can you imagine? You can binge-watch every season of Parks and Recreation, sleep for four days, and then simply turn back time and start studying! Like all good things in life, I’m sure the time-turners would be banned for this very reason.
Dumbledore would have confiscated fake IDs before the Triwizard Tournament.
College students are always trying to find a way to do things they aren’t old enough to do yet, so the ones who didn’t try an aging spell like Fred and George would use a fake ID instead to get their name in the Goblet of Fire. I’m not sure how those logistics would work exactly, but trust me, it would happen.
Slytherin House would get in trouble for hazing.
If the different houses were more like fraternities, you know that Slytherin would take traditions to the extreme and haze all of their new members after the sorting ceremony. The older students would probably petrify them, make them eat slugs, and probably even use the Imperius curse to make the pledges do their bidding.
No one would have shown up for Dumbledore's Army.
Every college freshman signs up for at least five different clubs and never attends a single meeting. It’s practically a rite of passage! Or, maybe they would attend the meetings at first, but once Harry started teaching them harder spells, a lot of the students would peace out to go play some Quidditch beer pong instead.