The inactive explosives were discovered in the engine compartment when the Loudoun County Public Schools bus was undergoing maintenance, the Central Intelligence Agency said in a statement on Thursday.
A CIA canine team left the inert explosives behind after a detection exercise held at a high school during spring break, it said.
The school district in the Washington suburb said the materials were aboard the bus on Monday and Tuesday when it carried elementary and high school students.
"The CIA assured Loudoun officials the training material used in this exercise is stable and posed no danger to students on board the bus," it said in a statement.
The inert explosives were inside a container and part of them apparently fell out into the engine space, the school district said.
The CIA said the material had been recovered and that it was taking steps to review the training and tighten procedures in its canine program.