Useless and the School Fete. A Useless Series, #3

Ewen "Useless" Euston is just trying to find his twin sisters at the school fete when he steps on a live wire behind the old fairy tree and vanishes into another world. The world looks like a fairy tale, but it runs on science. The elves in silver suits riding hover mats are intergalactic criminals who have been using the portal to abduct children. They have a quota to fill and a market off-planet, and sixteen kids are already trapped.
The portal decays at sunset. Every child still in this world when it closes stays forever. Ewen, class brain Will and resourceful Perry have one afternoon to remove implanted tracking devices, disable a force field, and get sixteen kids home. Their allies include a deposed dragon king merged with a lizard and a pterodactyl, a wizard, and his Halfling helper Tellick. Their enemy has hover mats, disintegrator weapons, and a very short patience for children who won't cooperate.
The kid they all call Useless is going to need everything that name never gave him.