Avalon: The Warlock Diaries by Rachel Roberts

Teenage mages, Emily, Kara, and Adriane have their hands full monitoring the portal at the Ravenswood Preserve. The portal has become the "Grand Central Station" of the Magic Web, with magical creatures and animals stopping by on their way to otherworldly destinations. Matters become really complicated when a new student, a handsome young warlock named Donovan, shows up at Stonehill High. When monsters start attacking Ravenswood looking for Donovan, the mages realize he may be trouble. However, Donovan holds a fairy map with a portal sequence that could lead the mages right to Avalon, the source of all magic.With the prospect of finally finding Avalon in their grasps, the girls try to work with Donovan towards their common goal, if mage and warlock magic can mix without putting all of Ravenswood in terrible danger!