Air strikes and troops that pass through town on a regular basis have shutdown the school and put poor post graduate Cheerilee out of the job. Her students had loved her when she taught them as a student teacher and were beyond enthralled when she was hired as the newest grade school teacher in Ponyville after she completed her degree. But it only lasted about a year. Cheerille couldn't manage the school by herself when the other teachers either moved away or were drafted as well (she has a heart condition that saved her from being drafted).
During the five years of the war, Cheerilee worked more as a babysitter and occasional tutor rather than a teacher. However, being a young and beautiful mare, she caught the eye of a few soldier who came through town. One of whom somehow managed to get her pregnant, but she won't tell anypony who it was or even if it was consensual.
Anyhow, nine months later, she gave birth to a little filly, which she named after her sister, who had died in her childhood. April.