Undead Series by Kirsty McKay

When their ski-coach pulls up at a cafe, new girl Bobby and rebel Smitty stay behind. They hardly know each other, but that changes when they see the others coming back. Something has happened to them - something bad. Soon, only a pair of double doors stands between those on the bus and their ex-friends outside.

The good news: Bobby survived her Undead school trip. Bad news: her best mate, Smitty, is missing. Bobby knows she's got to find him even if it means risking it all and going out into the starving-zombie-infested wastelands again. Even if it means taking fellow survivors including a couple of old frenemies along for the ride. And even if the zombies are not the only ones who are chasing them this time.