Here is the full list of all 14 Headmaster achievements.
Hit large goal crossbar 3 times in a row
Pass Exam 1
Receive the Snack Cart
Perform The Proper Heading Motion
Catch Robert in the Porta-Potty
Hit Robert in the head with a Knife Ball
Hit the loudspeaker with a ball
Bowl 3 strikes in a row
Pass Exam 2
Pass Exam 3
Pass Exam 4
Earn your Freedom
Ace Every Exam
3 Stars on Every Lesson