Mission 1: Interior of school. Few zombies. Meet up with Dr. Rogien, the principal. Head to the gym.
Mission 2: Push the zombies out into the back. Many zombies.
Mission 3: Push the zombies back to the field. Legions of zombies.
Mission 4: Fall back to the parking lot. Mow zombies over with vehicles.
Mission 5: Inside is where you will make your last stand.
Those will be the levels available in the game.