This is the 5440th most frequent Polish word.
szkolną
"Szkolną" means "school-related" or "of the school" in English.
The word "szkolną" is used as an adjective describing the book, indicating its relation to school.
Otrzymałem szkolną książkę od nauczyciela.
I received a school book from the teacher.
"Szkolną" acts as an adjective describing the bag, showing that it is related to school or education context.
Szkolną torbę noszę codziennie do szkoły.
I carry my school bag daily to school.
The term "szkolną" modifies the noun "wycieczkę" to indicate that it is a trip organized by or for the school.