It was the first day of the school year. Mr Bubble , the Principal of Kansas City High School, was brutally murdered.

The police had 4 suspects: the gardener, the maths teacher, the PE teacher,  and the English teacher. They all had alibis:

• The gardener was cutting the grass in the school playground.
• The maths teacher was doing a geometry lesson in his classroom.
• The PE teacher was playing basketball with some 8th grade students in the gymnasium.
• The English teacher was marking a test he gave to his pupils the week before.

The police arrested the murderer immediately. Who killed Mr Bubble? Why?