Dragon in School (en Inglés) - Anthony Robinson

John Lewis, Beelzebub the Dragon Lord, the young wizards, (Terrence, Digby, Nigel and Sven), his witch cousin Serena from the future, his witch cousin Alicia from Iteria and his witch cousin Sybil Robinson from Coventry had just finished defeating a nasty warlock named Suleiman the Magnificent who had came from Earth to the planet Aspiro and was at least 500 years old. On Earth, Suleiman was a ruler of about 25 million people, and he used his wealth to collect enough magical spells to enslave the earthly dragons. Then he used their magic to make himself young again, and he also used dragon magic to travel to the various worlds in the known universe collecting more magic spells.

After the warlock's defeat, Terrence, Nigel and Sven voted to go to the future and help Serena and the Twins rebuild their world.
Alicia and Digby chose the task of building up magic in Iteria to prevent more attacks by rogue wizards.
Sybil and John just wanted to go home and become ordinary school children again.
Beelzebub and his dragons wanted nothing to do with humans again.
But time is a strange entity, and so Sybil and John could not stay ordinary children, and Beelzebub could not stay away from John.