This is the 18306th most frequent Russian word.
школьного
"Школьного" means "school-related" or "of the school."
Here, 'школьного' is an adjective derived from 'школа' (school) and is used in the genitive neuter singular form to describe the backpack, indicating that it belongs or pertains to school.
Я не могу найти своего школьного рюкзака.
I can't find my school backpack.
In this sentence, 'школьного' again acts as an adjective describing the class (in the genitive neuter singular form), denoting that it pertains to the school.
Это фотография нашего школьного класса.
This is a photo of our school class.
Here, although 'школьную' is a declined form matching gender and number, the base adjective 'школьный' still reflects properties associated with the school.