A lot on how you want the mundane world to see the school and where you want the school located.

If you wish the mundane world to not even know it exists you could use something like the Cherokee legend of the village of the Yunwi Tsunsdi' that live under the mountains and have the school located in one of these magical areas that the legends state these magical people live. Going with the legend, since the school would be in one of the under the hill magical places, a name like Underhill Academy would work fine. I kind of like this idea since it would give you a ready made race of teachers that look like little children instead of teachers and staff.

If you use this, the Yunwi Tsunsdi' are normally said to look like children but only 2 feet tall or smaller, with long hair that almost touches the ground. They are said to be benevolent and also area also able to become invisible. of course to stick with the legend you would want to place the school in the Appalachian mountains in either Tennessee, North Carolina, West Virginia or Virginia.

Now if you wish your school to appear completely normal and mundane but not have any non magical families trying to enroll their children, then you would need the school to appear as something normal mundane parents would not wish to send their children to. So a reform school or school for troubled children or some sort. This would give you a reason to have prison like fencing, barbed wire, guards patrolling the perimeter, etc. Which would look to anyone viewing it from the outside as measures to keep the students from escaping when in fact it is to keep anyone from entering that should not. Then of course use whatever name strikes your fancy for the school and add "Alternative school" to the end of it. such as "Alexandria's Alternative School"

Something like this alternative school could actually be located in the middle of a city if you chose to do that.

Now if you want the school inaccessible and hidden, the easiest thing is to put it on an island. Plenty of islands off the coast of California if you wish west coast. East coast North Carolina has the outerbanks islands which also give you the benefit of Blackbeard having used those islands as his stomping grounds and that it is also known as Davey Jones locker. Both of which could lead to some interesting plots that could develop in the story. Then you have the Florida keys and the Caribbean, so full of islands take your pick. As it's not that well know for people that do not live in Florida, there are a chain of Keys going up the gulf side of the state too. Also in Florida you have the Everglades, great place to hide something there and with any of these a little magic to keep anyone from being able to see it...