Why Should I Afraid

Once all the engineering professors were sitting in one place. Before the take-off, one announcement came, this place is made by your students. then all professors stood up, ran, and went outside. but the principal was sitting. one guy came and asked, are you not afraid? then the principal replied, I trust my students very well, I know about them very well and I am sure the plane won't even start. then why should I afraid.