Loudoun schools spokesman Wayde Byard said the CIA indicated the nature of the material but asked the school system not to disclose it. Byard described it as a “putty-type” material designed for use on the battlefield and which requires a special detonator; such putty, or plastic, explosives — including the well-known C-4 — are used in demolition and are considered stable.Stable is not "inert"--it was an explosive that but lacked a detonator.
Also, plastic explosives are flammable and produce toxic fumes when burned. Thus, while there was no real risk of detonation, it wasn't as safe a situation as the officials pretend, either. If it had fallen onto a catalytic converter, exhaust manifold or exhaust pipe, it could have caught on fire.
Of more concern, though, is why there were no protocols in place to ensure that the same quantity of explosives checked out at the start of the exercise were not checked back in at the conclusion.