How does Chell in Portal get inside Aperture Science in the beginning of the game

Though there are signs in Aperture Science halfway through portal 2 mentioning,"bringing your daughter day" That was the supposed day GLaDOS took over the facility. It shows the science projects made by the girls on bring your daughter to work day. majority of those are potato batteries, the one on the end that turned into a giant potato plant, is signed by Chell showing she was a daughter of an employee at Aperture Science.