A little more on the Urban Military Academy.
This summary is from
"The American Pre-College Military School: A History and Comprehensive Catalog of Institutions" by Samuel J Rogal.
books.google.com
This is definitely the same location as
e_r's aerial view.
Boys riding horses toward campus buildings, Urban Military Academy, Brentwood, Los Angeles.
Huntington Digital Library
Here's one of the LAPL aerials posted by above by
Tourmaline. It shows the Urban Military Academy on Melrose Avenue at Wilcox.
LAPL
The Los Angeles Tennis Club now occupies that site. I think that the tiled building under the word "Club" may be a survivor.
Google Maps