This achievement must be obtained either during your first visit to the school, during the main story quest to free Craig, or during your second visit when you assault the school. During the first visit, numerous Gingers can be taken out with the environment, and the game will prompt you to do so. During the second visit, however, the Gingers will be zombies, and therefore much more likely to bite you (as opposed to ranged or weapon attacks), so it might be easier to farm this achievement during your second visit.
I recommend making a save as soon as you enter the school. Go through the school and every time you fight Gingers, let them attack and block each attack until they perform Ginger Bite. Don't block this attack. Let it happen 3 times to get the achievement, then revert your save so you don't waste any potions you might have used while waiting for Gingers to bite you.
NOTE that if a Ginger bites your ally instead of New Kid, this still counts toward the achievement.