You must rescue all 21 classmates in the game for this achievement. Classmates will have an SOS next to them to indicate that they are savable. To check your progress press then to see how many you have saved for each stage. If you happen to miss a classmate fret not, as you can run back and pick up any ones you missed upon completing the game! Here are the amount of classmates to be saved per stage:
Prologue: 3 Classmates
Stage 1: 8 Classmates
Stage 2: 4 Classmates
Stage 3: 1 Classmates
Stage 4: 3 Classmates
Stage 5: 2 Classmates
If you don't miss any on your 1st playthrough this can be unlocked by Stage 5 (Cathedral). The final 2 SOSs are in the Cathedral. They are always in the main game path and they are not too hard to save as they are usually in plain sight. The main thing is to make sure you actually go up to them after you save them so that you get credit and the Saved pic shows up in the bottom right of the screen. Each classmate will reward you with medals upon saving them. Below are some of the missables:
There is one on Stage 3 - there is a blastable boulder to the right of a SOS student. If you shoot the boulder, it will instantly kill the SOS so be careful there.
The last one at Stage 5 is the only hard one. You have just closed off a well so that bomb zombies stop coming up. Then you'll get a call of about a puppy. Once the call is done quickly shoot the barrels with your blaster to get rid of them or you'll kill the SOS guy.
fairly simple since it does not have to be in one run. you can save one classmate and if you miss a diferent one you do not need to resave the other one when you go back through. but honestly i save them anyways just for the coins and funny comments.
the only ones to be careful about are the three in the classroom in the high school they have bombs on them and you do the poledance thing, i actually did not kill the 12 zombies one the pole the first time through so one got by and blew them up. just be wary of that id suggest waiting like 3 seconds before jumping on the pole.
So just to make sure I understand this. If there are 3 students and I only save 1 and 2 this first time through. Then on my next run through 1 and 2 happen to die and I save 3 it will still count for all of them?
#4, it seems as if you do have to finish the level. i quit after i saved the last one i needed on level 3 and i didn't get the happy ending option when i beat killbilly again. i got the achievement for saving all the students when i saved the last one, but by quitting the level after doing that, it didn't fully register.
I'm missing the fourth survivor in stage 1, is this the girl whos hanging from a window ledge and then gets pulled through by an exploding zombie? Because thats happened on both my playthroughs and it seems like it needs to happen to break the wall so you can progress. =/