Rex leaves the principal's office after talking to each other.
The principal is like a man of mystery for Rex, it's like every time he does something he felt that the principal might be watching.
Think of a shadow, Rex can feel the principal is right on his back.
'What is he planning? He let me go just like that', Rex thought.
After he and the principal argue about the spellbooks, the principal suddenly lets Rex go without even questioning him further.
Rex knows that the principal knows he has the spellbooks, so it's weird for him to let Rex go.
Leaving the principal's office, Rex immediately went out of the university fearing that there's someone following him.
He doesn't feel safe anymore,
The principal can just teleport him out of nowhere without even him noticing, it's not a great feeling for him to get teleported so easily like that.
After getting out of the university, Rex went another way.