This is the 15746th most frequent Polish word.
szkolny
"Szkolny" means "school-related" or "pertaining to school."
Here, 'szkolny' modifies the noun 'plecak' (backpack) to mean that the backpack is associated with school, likely intended for carrying school-related materials.
Mam szkolny plecak, który jest bardzo wygodny.
I have a school backpack, which is very comfortable.
In this case, 'szkolnym' is the locative form of 'szkolny', modifying 'ogrodzie' (garden) to denote its association with the school.
W szkolnym ogrodzie rośnie wiele roślin.
In the school garden, many plants are growing.
The word 'szkolny' here describes 'autobus' (bus) to specify its function or affiliation as being related to school transportation.