Located in Valley Township
Valley Township was formerly Columbia County, Now Montour County PA
See a township map here - http://www.usgwarchives.org/maps/pa/county/montour/usgs/
(I'm in the process of collecting information on the Sidler School house, in my efforts to learn more about Christianna Sidler who married James Crossley. We know that Christianna's son Joseph attended the Sidler School house, so there's a good chance there is a family connection)
George B. Kase
"Andrew J. Wintersteen was educated at the Sidler schoolhouse in Valley township"
Joseph H. Crossley attended Sidler School house, he was born in 1817.
"Joseph H. Wintersteen attended school more or less regularly until he was nineteen years of age, in the Sidler schoolhouse in Valley township, and among his teachers he recalls very kindly Martha Crossley and George Kase." Born in 1863
George Bonham Kase was born in Illinois in 1859, making him just 4 years older than Joseph Wintersteen. In 1870 George was age 11, living with the kilmers, and a variety of what appear to be Kase siblings in Illinois. In 1900 he is found in Montour County PA. I do not know what years he taught at the school, but since Joseph would have been 10 in 1873, and 18 in 1881, I assume he taught there sometime between 1871 and 1881.
The only Martha Crossley I find in Valley Township Montour County, was born in 1826 as Martha Susan Wilson, and was the wife of Reuben Crossley. It seems unlikely that she would be teaching while still having children, which she did from 1849 to 1861 - but I do not know.