This is the 3782nd most frequent Russian word.
школьную
"Школьную" means "school-related" or "school" (feminine singular accusative form).
The word 'школьную' is an adjective modifying the noun 'сумку', indicating that the bag is associated with school.
Я забыл школьную сумку дома.
I forgot my school bag at home.
Here, 'школьную' serves as an adjective describing the noun 'стену', specifying that the wall is a part of the school.
Мы красили школьную стену вчера.
We painted the school wall yesterday.
In this sentence, 'школьную' is used as an adjective qualifying the noun 'форму', referring to the uniform being related to school.