The full cite is Rundquist, E. A. (1936). Intelligence test scores and school marks in 1928 and 1933. School & Society, 43, 301–304.
The article Lynn, R. Who discovered the Flynn effect? A review of early studies of the secular increase of intelligence, Intelligence, 2013, 41, 765-769 states that Rundquist’s 1936 article is the first (so far as is known) to find increases in average IQs.
That might be an interesting read, but it is not easy to find (so far).