Will I be able to do this? What if I bring it and they do not allow it?
Why not check with the school first? 
The school does not say that I’m not allowed a desktop, but I am definitely sure that they do no allow monitors larger than 20". @ChoatieMom
I doubt that there is an issue bringing a desktop, but all schools will limit the monitor size. As @ChoatieMom said, just ask the school.
The bigger question would be why you would want a desktop. (The question is rhetorical, so don’t answer.) Most students have a laptop, and if your school requires a laptop, it seems silly to bring a desktop. But I have to use a plane to get to my school, so I’m a huge proponent of packing light. YMMV.
Schools limit desktops screen sizes often. The enforcement can be hit or miss because you will see quite large displays quite often, typically in boys’ rooms as they are used for gaming purposes.
Don’t get sucked into wasting your time gaming at boarding school. It’s a waste of a great opportunity and your parents’ money.
OTOH, CK shipped his desktop to school his first two years because he needed more heavy-duty graphics processing (for film production) than his laptop allowed. Eventually, laptop technology/software caught up at a reasonable price and he was able to travel lighter.
The points made by @skieurope and @doschicos bear repeating: Travel light and DON’T waste any time gaming at BS.
I have a desktop, laptop, and an iPad. But now my laptop is kind of breaking down as it’s about six years old. I’d recommend you bring two computeresque devices though, in case one breaks down and you’re stuck without for a month before the Apple Store has a reservation open.
@mrnephew, where do you live that it takes a month for a reservation at the apple store?? Near me, you can get one tomorrow, or take it to Best Buy. Also, at many BS the tech folks will help you try to fix it. You can also utilize the free library/dorm computers in a pinch or borrow from friends.
Having 2 devices, let alone 3!, is unnecessary and excessive. You also run the risk of classmates rolling their eyes about your privilege.
I would not go that far, especially for 2. At some schools, an iPad is required. Even if not, a tablet does come in handy at times when a laptop might be too intrusive, e.g. in class (with teacher permission of course).
Desktops can sometimes be less expensive than laptops for comparable. if you already own a desktop it might make sense to bring it as long as your monitor fits the size requirements.
Actually… Running the risk of classmates rolling eyes at privilege is highly unlikely at a top tier school - a good percentage of people have three devices, if not more.
Some people bring 45 inch monitors for gaming… Having a laptop, a desktop, and an iPad isn’t nearly as ostentatious.
My Apple Store reference was hyperbolic, but going to the nearest Apple Store or best buy may be difficult if you don’t have a ride.
Clearly I’m missing something here, but what is the parents’ rationale, after going through 65 page packing list thread, for saying, “Our son needs to have a 45” monitor at boarding school?"
I’m not condoning it, just providing info.
What laptop is most popular at Lville?
Granted, there are plenty of wealthy kids at boarding school, and some do have multiple devices, but, yes, flaunting wealth does get comments and eye rolls.
There is also a difference between kids bringing multiple devices to school and posting here, where many posters are on FA, RECOMMENDING that its a good idea to bring multiple devices. It just isn’t necessary.