undefined method `school_users’ for nil:NilClass

Estou com o seguinte problema:

def index  @school_users = current_school.school_users. includes(:user => [:user_roles => :role]).includes(:school_grades). includes(:discipline_school_users => [:community, :discipline]).select{|s|   s.teacher? && !s.master? }.sort{|a,b| a.name <=> b.name}  @school_users.each do |scu| scu.delete if scu.user.active == false  end   end