What is the level of the course with the most number of teachers?
level of the course refers to courseLevel; course with most number of teachers refers to course_id = max(count(taughtBy.p_id))
SELECT T1.courseLevel FROM course AS T1 INNER JOIN taughtBy AS T2 ON T1.course_id = T2.course_id GROUP BY T2.course_id ORDER BY COUNT(T2.p_id) DESC LIMIT 1