If Athena stayed in bed, she'd miss her chance to direct the power of the magickal energies through her, so she could cast a spell to make Sarah McMillan her completely obedient slave. Sarah was about the closest person she had to a mortal enemy and she couldn't wait until this day when she was finally going to be able to put that bitch in her place.
So, she got out bed and began to get ready for school. 8:42 AM was around the end of her first period class and she needed to be close to Sarah, who fortunately (for this) was in that same class, so she needed to go to school.