Note.rb

#...
  has_many :notes_students
  has_many :students, through: :notes_students
#...

notes_controller.rb

    assigned_note_ids = NotesStudent.select("DISTINCT(note_id)").map{|ns| ns.note_id}
    @notes = current_school.notes.where("id NOT IN (?)", assigned_note_ids).page(params[:page])

notes_student.rb

  belongs_to :note
  belongs_to :student

student.rb

#...
  has_many :notes_students
  has_many :notes, through: :notes_students
#...