When [the student] is "off," our school day is often horrific. She certainly doesn't learn a damn thing on those days or weeks. ... I should not be made to babysit kids who are prescribed medication, and are left without by negligent parents. It would be different if her mother had doubts about medication overall, that kind of thing. No, it's laziness.
Have you tried to discuss this issue with the parents? Have you asked an administrator for help problem-solving.
Maybe the family is under a lot of stress in the mornings and they would agree to have the school nurse administer the meds in the morning at school.