Could this be "Midrealm" by Garrett Robinson?

What if you and your friends went to another world every time you went to sleep?

What if you spent half your life here, in our world, and the other half battling for mankind’s survival against foes too terrible to be real?

And what if you could never tell anyone?

This is reality for the Realm Keepers: six people from Earth thrust into a terrifying war between Midrealm and Chaos. Plucked from their ordinary lives and given extraordinary powers, they’re the only thing standing between our world and destruction.

...

Midrealm is a story about six teenagers who are in detention one day and pass out. They wake up in another world, where they are the Realm Keepers, wizards with incredible powers. Each Realm Keeper has a different element to control. The world is threatened by an evil wizard called Terrence that they need to battle, to save both Midrealm (the fantasy world) and Earth.

Could this be "Midrealm" by Garrett Robinson? It's much too late (2013) but the plot sounds astoundingly similar...

What if you and your friends went to another world every time you went to sleep?

What if you spent half your life here, in our world, and the other half battling for mankind’s survival against foes too terrible to be real?

And what if you could never tell anyone?

This is reality for the Realm Keepers: six people from Earth thrust into a terrifying war between Midrealm and Chaos. Plucked from their ordinary lives and given extraordinary powers, they’re the only thing standing between our world and destruction.

...

Midrealm is a story about six teenagers who are in detention one day and pass out. They wake up in another world, where they are the Realm Keepers, wizards with incredible powers. Each Realm Keeper has a different element to control. The world is threatened by an evil wizard called Terrence that they need to battle, to save both Midrealm (the fantasy world) and Earth.