The Demons of Paris

It's January of 1372 and the space-time continuum has been breached. Demons of all kind, evil and benign, spill from the netherworld into the human one.
In Paris, a series of grisly murders that couldn't be performed by a human, no matter how depraved, leads the Grand Chatelet to try and raise a demon of his own to combat whatever monster is terrorizing the city.
Unfortunately-or perhaps fortunately-the demon who is summoned brings with him a van from the Paris of the twenty-first century. The van is filled with a drama teacher, her son, and eight high school students-along with all of their electronic devices.
Soon, more demons are summoned, all of whom are entranced by the electronics of a future world. Laptops, tablets and cell-phones-not to mention the van's equipment-become possessed by imps and spirits of the netherworld, some of whom are brilliant and all of whom are curious.
What could go wrong?
And King Charles V had already been in trouble! Piled onto his own poor health, a suspicious and contentious church, France's always-quarrelsome nobility-worst of all, his unscrupulous and ambitious brother, Philip the Bold-the king now has both demons and people from the future to deal with.
He does have one asset-and not a small one. He can place his trusted Constable of France, Bertrand du Guesclin, in charge of the rambunctious teenagers from the future and their ever-growing legion of demons. And Bertrand has a great asset of his own-his wife Tiphaine de Raguenel, perhaps the best astrologer in all of France and, for sure and certain, not a woman to take seriously the prattling nonsense of youngsters skeptical of her lore and knowledge.