Full Moon Halloween by R.L. Stine

You hold in your hand the key to a shadow world of shivers and screams. Take a step away from the safe, comfortable world you know. Unlock the door to terror. There's always room for one more in...The Nightmare Room.

Mr. Moon is throwing a Halloween party, and Tristan and his best friend Rosa have been invited. How lame Tristan thinks. He'd rather be trick-or-treating than hanging out with his teacher

But Mr. Moon's party is far from lame...because he's got a couple of werewolves on the guest list Can Tristan escape the party? Or will he find himself trapped by a werewolf in The Nightmare Room?