There are many reasons to want to live in the world of My Hero Academia: Quirks, heroism as a career path, and of course, the illustrious school of U.A. itself -- but with the current world situation, mostly people want to escape to a world where COVID-19 simply doesn't exist. But if COVID-19 did hit the world of heroes, how would they deal with quarantine?
Well, for those at U.A., not much would change at all — they're basically in quarantine already! After all, Season 3, Episode 50, "End of the Beginning, Beginning of the End" and Episode 51, "Moving into Dorms" turned U.A. into a high-security boarding school. This means that all the students live on campus, and can access to the school's many amenities and teachers at any time. When you take all these points together, you quickly begin to see why U.A. is the best place to be spending quarantine.
First and foremost are the teachers. With Recovery Girl acting as nurse, no matter the injury, there's no need for any student to go for outside help. Broken arm? Smooch! Allergies? She can order medication for no-contact delivery — stat. With her age and experience in addition to her Quirk, she's more than equipped to handle any medical emergency.
But the teachers' ability to care for their students goes beyond healthcare. Lunch Rush has been providing nutritious and delicious meals for the entire campus, and his hero costume/chef outfit is already quarantine-approved. With the tube connecting his face to his back (we assume to an oxygen tank or something), you can be sure that he won't be breathing on anybody's lunch. If needed, he could also use the school's mini transport robots to deliver food straight to each class's dorms. So long as food-safe regulations are followed by the delivery drivers, the students will never need to worry about their next meal.
Speaking of dorms though, each class has it own building. Each one fits just twenty or so people, complete with its own laundry, baths and dining hall. While the whole school is already quarantined thanks to the barrier that prevents all non-faculty and non-student personnel from entering the grounds, if anyone was diagnosed the campus could go one or even two steps further. They could self-isolated the affected individual(s) to their rooms or the nurse's office, using holographic technology to present lessons should they feel up to it, and get each class to isolate in their dorm, having non-combat classes in the dining hall, where the students could use tables instead of desks. With the variety and space of the various training areas, the school could even assign each class one training area, allowing the more practical classes to continue. Sure the students will get to know the layout of the area well, but it will work as a good practice for working on "home turf" when they finally do strike out into the world.
And finally, the biggest reason no one would need to leave the grounds: Momo Yaoyorozu. Even though she's just a student, she would love to assist, and would probably take every precaution available to avoid the illness -- so she can keep creating N95 masks for her fellow students and the tools needed to fix any minor malfunction, whether it's for the support class or a fellow student whose broken the light in their room practicing their Quirk for the tenth time, or even just making disinfectant and industrial sprayers to ensure cleaning and sanitizing is as easy as possible. Not only does this allow her to be the school hero, but it gives her bucket-loads of Quirk practice and practical experience for any future bio-warfare occurances.
But all that said, there's one more reason why U.A. is the best place to quarantine -- because isolation is always better...together. From Deku to Tsuyu to even Bakugo, staying apart to keep your loved ones safe is important -- but so is one's mental heath. By isolating in their dorms, people who they've all been in close contact with already, the students are free to keep hanging out together so long as everyone does their part, and wears masks when outside the dorm. They can still play together, and more-or-less continue living carefree, because they know that as soon-to-be-heroes -- they're all doing their part.
So basically, should COVID-19 hit the My Hero Academia universe, a school-wide isolation is the most reasonable, effective and immediate stance for U.A. to take. Of course, villains don't tend to care about little things like pandemics, but...the likelihood of those showing up a third time are low. Right?