Why Sam allowed to attend classes at the school along with other men in town?

Sam was not allowed to attend classes at the school along with other men in town. Sam is the protagonist in the novel "The Absolutely True Diary of a Part-Time Indian" and is a Native American boy who lives on the Spokane Indian Reservation. He is from the fictional tribe "Wellpinit". Due to the tribe's poverty and lack of resources, Sam is forced to attend an underfunded public high school off the reservation.