This is the 18307th most frequent Russian word.
школьным
"Школьным" means "school-related" or "pertaining to school."
In this sentence, 'школьным' is used as an adjective describing 'friends' to indicate that they are friends from school (school friends).
Я помог школьным друзьям подготовить презентацию.
I helped my school friends prepare a presentation.
Here, 'школьным' functions as an adjective characterizing 'uniform' to specify that it is associated with school (school uniform).
Мой брат носит школьным комплектом одежды каждый день.
My brother wears his school uniform every day.
In this context, 'школьным' describes 'event,' specifying that it is an event related to school (school event).