To obtain the Good Ending you must complete Stage 6 (Killabilly), while having all of the classmates rescued. If you happen to miss some then you will receive the Bad Ending instead. Don't fret if you get the Bad Ending. You can simply use the Stage Select option to run back and save any classmates you missed! See San Romero Knights Savior for a guide on how and where to save all 21 classmates. Once you have all 21 rescued, re-challenge Killabilly and complete the Stage to finally receive your good ending.