Flea darts are basically grass darts which, when looked at closely, sometimes have little black creatures moving about in them. These, of course, are fleas.
Like the purple headed mountain and each little bird that sings, they were invented by God. He designed them to stick to schoolgirls' white socks, who would then walk briskly about a bit on the way home from school before thinking 'Ugh, a flea dart'. They would then remove the flea dart, allowing it to germinate, giving rise to a new flea dart plant.
Flea darts are harvested at the morning break, and distributed into peoples' hair throughout the day. They are generally thrown at girls because they have long hair, or boys with curly hair, to ensure maximum attatchment effectivity.
Once a person has been "infected" with a flea dart, it is best to spend the rest of the day running away from them, as fleas are highly contagious and make a convincing argument that the person is dirty and smells.