during the school years

Вот, наковырял:
https://books.google.com/ngrams/graph?content=in+school+years%2Cin+the+school+years%2C+during+the+school+years&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cin%20school%20years%3B%2Cc0%3B.t1%3B%2Cin%20the%20school%20years%3B%2Cc0%3B.t1%3B%2Cduring%20the%20school%20years%3B%2Cc0

+++Tatiana

...not DURING THE SCHOOL YEARS

“during the school years” refers to a developmental stage in children. When we tell a story about something that happened while we were students, we simply say “(back) when I was in school” or “in my school days” or simply “in school”

To me, в школьные годы sounds as a generalization. Именно в школьные годы человек делает большой скачок в своём развитии. If I refered to the specific period of time I was in school, I'd say в мои школьные годы or когда я училась в школе/мы учились в школе.

In that case, it is in fact DURING THE/ONE’S SCHOOL YEARS

However, someone can use в школьные годы to refer to their time in school. So, context, context...