Penn does not have an official policy on excused or unexcused absences except for religious holidays.
Penn's Policy on Secular and Religious Holidays asks that instructors excuse students students observing religious holidays that affect large numbers of students, including Rosh Hashanah, Yom Kippur, Christmas, the first two days of Passover, and Good Friday. Exams scheduled immediately after those holidays should not include material introduced on that day.
Student Health Services will not provide notes verifying student illnesses. You should design policies that do not require medical confirmation. Should a student need to miss an extended period of class (more than three days), you should contact the student's advising office.