Write one or more triggers to manage the grade attribute of new Highschoolers
As i mentioned in the comment.

For your purpose you can use a BEFORE  INSERT trigger

DELIMITER $$
CREATE TRIGGER `gradeTrigger1` 
BEFORE INSERT 
ON highschooler  FOR EACH ROW
BEGIN
    CASE  WHEN  new.grade &lt; 9 OR new.grade &gt; 12  THEN
        SET new.grade = NULL;
    WHEN  new.grade IS NULL THEN
        SET new.grade = 9;
    ELSE
        SET new.grade = NEW.grade;
    END CASE;
END$$
DELIMITER ;