Quick search found this page but paywalled. Thesis is focusing on Korean education during the annexation. It's anyway 11 years old, there might be more recent studies on this area.
Quick summary for those who doesn't have access:
The Language education during Korean annexation by Japan can be split to 3-4 stages (author stated he will base his thesis on 3 stages): first for 1910-1919 (after the annexation until 3.1 movement), second for 1919-1938 (after 3.1 movement until Japan's war with China), the last for the remaining (1938-1945).
All textbook's language is Japanese except Korean and Hanja reading. 'National language' is Japanese, more time is allocated for Japanese, and even in 'Korean and Hanja reading' class, most time is devoted for Hanja, resulting less time for Korean. (Japanese education was conducted outside the school, too.) 340K people knew Japanese, around 1.6% of men and 0.2% of women.
1922 Chosun education order split 'Korean and Hanja reading' inti 'Korean' and 'Hanja', but 'Hanja' time was practically Kanji class. It also split the course for Japanese in Korea and Korean. Usage of Korean was banned in school, you may be punished (physically or financially) for violation. Outside the school, Japanese class was also promoted, while Korean class was often disturbed by Japanese police. 12.4% of Korean population spoke Japanese in 1938.
1938 Chosen education order made Korean class optional, and all classes except Korean be conducted in Japanese. Even if schools may decide to teach Korean with the class being 'Optional', Japan tried to interfere them. All Koreans were forced to change their Korean name to Japanese, all Korean newspapers and magazines were ordered to cease publication. Approximately 27% of Korean population were expected to speak Japanese by early 1945.
While this is not the complete answer, this should give you some glimpse on the subject.