I wondered the same thing eat_the_pudding.
The BBC coverage suggested he wasn’t removed from class and detain until later in the day.
The teenager said another teacher became aware of it when the device beeped during the lesson.
“She was like – it looks like a bomb,” he said.
The homemade clock consisted of a circuit board with wires leading to a digital display.
Later in the day the boy was pulled out of class, interviewed by senior teachers and four police officers, and put into juvenile detention.
Surely if they genuinely thought it was a bomb they would have detained him immediately and evacuated.
All in all it’s a pretty good example of why the NRA’s plan to give teachers guns is more than a little flawed!