The most reliable date I've found for the garage itself was in the 1963 yearbook, which states that the garage was built in school year 1939-40. I scanned this from an unlabeled print, which appears to have been the one used in the 1941 yearbook. The photo in the 1939 yearbook shows only the buses, and was taken from the opposite direction with the Morrow house in the background.
Still searching for drivers' names.