For the following reasons, this is a big problem; schools, even former schools, are in high population areas.
The odds of being seen are extremely high, and you'd be seen by curious children or parents. I dont want to think about the shit that would follow.
If you rented the building, you must allow for security, cleanup, damages, etc.
If you dont have a contact with the local police to help you deal with paperwork and legalities... I sure as hell would not even try.
I've seen, in person, a full SWAT team react to a really stupid call. If a member of the public can mistake a guy dressed in 18th century clothes and carrying a flintlock musket for a "GUY WITH A SHOTGUN!!!" (Yes, it's true), then what we play with is sure to give them a heart attack.
Why dont you look around for a former warehouse, old office building or old factory instead? If it's out of view, bonus. Those exist in most cities/suburbs.