This is the 3272nd most frequent Polish word.
szkolnym
"Szkolnym" means "school-related" or "pertaining to school."
'Szkolnym' is an instrumental form of the adjective 'szkolny', describing the noun 'ogrodzie' (garden) with the associated concept of belonging to or related to school.
W szkolnym ogrodzie rosną piękne kwiaty.
Beautiful flowers grow in the school garden.
The word 'szkolnym' is an adjective in the instrumental case, modifying 'plecaku' (backpack) and meaning 'school'.
W szkolnym plecaku mam zeszyty i książki.
In the school backpack, I have notebooks and books.
'Szkolnym' modifies the implied noun in instrumental usage referring to something related to the school.