Clocks Don't Alarm People
My guess as to what happened: Ahmed built the homemade clock, took it to school, and showed it to his engineering teacher. After receiving some encouraging words, he went to his next class and said “Look at what I made!” The teacher there heard that as a threat.
As for Ahmed's reticence, I understand that is the appropriate response to police interrogation nowadays. Maybe his parents are Martha Stewart fans and he heard of what happened to her.