[6] STA became a U.S. Army school in 1905, and, in 1916, part of the Junior Reserve Officers' Training Corps (JROTC).

The Corps of Cadets, as the high school students are sometimes referred to, is inspected by representatives from the National Guard in the springtime.

In addition to checking their formal uniforms, the representative asks a few questions, most of which concern the school, its history, or U.S. Army.

In the spring, the high school student body dons their formal uniforms (Class A1) for the Archbishop's Review, in which the students march around the school's track while being watched by the Archbishop of Minneapolis and St. Paul.

Saint Thomas Academy was a member of the Classic Suburban Conference (now Metro East).