Anything ?

In order to get the happy ending you need to rescue all of your schoolmates with the SOS over their head. Otherwise you get the bad ending. Rescuing all of the students comes with it's own achievement, so when you see that pop up finish the chapter you're on and then complete the last chapter.

Like ford817 said help all the students with and sos over their heads but you will have to escort some to safety while otehrs you can just walk up to and watch them fade out after they give you gold zombie medals.