Covid has had an impact, but before, the field house at Lawrenceville was pretty much always open during “regular hours” (7 AM - 8 PM).
Dining hall is only open during mealtimes, but the school store and Bath House (on-campus café) have different hours; also, you can always go to Main Street to buy stuff. Many Houses also have concessions stands where boarders can buy food during study hall (8 - 10 PM).
I’m not under the impression that any school lets students out of their dorms/the library/other study spaces past 9 PM on weekdays, by the way.