Tim Worstall points to this interesting paper by BLS economists Ian D. Wyatt and Daniel E. Hecker.

Teachers below the college level increased 1.4 times as a proportion of total employment between 1910 and 2000, from 1.6 percent to 3.8 percent. The elementary and secondary school pupil-to-teacher ratio dropped by more than half, from about 35 in 1910 to 16.4 in 2000.

The chart in the paper suggests that most of the increase in educational employment occurred between 1950 and 2000.

The article is mostly about changes in the composition of employment from 1900 to 2000. The biggest reductions came in farming, which dropped throughout the century, and “operatives” (manufacturing workers), which rose until 1950 and then plummeted thereafter. The biggest increases came in professional and technical work and service work in industries such as health care.