Thanks, Ross. It does look like a school building to me, too.
However, I see in this Feb. 23, 1919, Houston Post article (p.34, pasted below) that the Montrose School appears to be two-story, not three. Was there another one on that campus. (In several articles about the Montrose school, I see mention of an administration building, but I haven't (yet) found if this news photo shows that ... or other photos that specifically show that building or say if it's 3 stories high.)