I found some pictures showing what took place inside the walls of the Frank Wiggins Trade School.
"Lecture for printing students at Frank Wiggins Trade School, 1920-1929"
USC Digital Library
"Instructional exhibit by Frank Wiggins Trade School, showing students and teacher, ca.1920-1929"
USC Digital Library
Here's an enlargement of the sign on the right. The only years between 1920 and 1929 that would fit these dates are 1921 and 1927.
Detail of picture above.
"Instructional exhibit by Frank Wiggins Trade School, showing students and teacher, ca.1920-1929"
USC Digital Library