Perhaps the "teacher" had the bomb stashed in his car and the heat of the day set it off.
Nail bombs aren't used inside cars, they are used outside in crowded places to inflict max damage. Ball bearing bombs are used inside cars to kill the occupants.
This teacher perhaps never had a chance to place the bomb, and was waiting till school was out to finish the job.
I wouldn't rule out the teacher as a suspect just yet. Search his house for bomb making materials.
Yeah, you’ve got to treat the owner of the car as a suspect, but so too could a car detailer/washer or car repairman have access to the trunk or some inner panel in which to hide a bomb.
Used cars in Florida are constantly having drugs found inside such areas by new owners...sometimes months after the car has been purchased. Lots of empty spaces hidden inside panels in most cars.