It's wrong to try to be someone you are not.
Jill ran the wrong way and scored a safety against her own team, thus she dreads going to school the next day.
Jill wants to avoid embarrassment for her mistake.
Jill decides to practice in the gym all day to avoid the cafeteria and the playground.
Jill pretends to be another student in her class to avoid embarrassment.
Jill is able to avoid the other students until the event is mostly forgotten.
Jill is eventually found out and is far more embarrassed with this then the original mistake.