Why would he think (and he doesn't, by the way) that removing clock guts and putting them into something else constitutes making a clock and worthy of achievement to show ... well, anybody?
That said, why didn't he use the exterior of an old clock to house it in? Or a lunch box or a cardboard box or ANYTHING that didn't look like a bomb?!
We all know why. This was either a dry run or an attempt to make reasonably cautious people look like Muslim haters.
I say dry run.
They cry victim and islamaphobs so that next time people are afraid to confront him so they back off.
Then the real bomb goes off and students and teachers, and AMERICANS die.