If a student is absent more than two days from a yearlong credit course, regardless of the circumstances, s/he will be automatically un-enrolled from the course and a "Withdraw" will be noted on the transcripts. For a semester-long class, students may not be absent more than one day, with the same consequences. See below for information from the Brentwood School Upper School Family Handbook.
"Summer Session: Students have until the end of the second day of the Summer Session to drop a class without penalty. After the end of the second day, Brentwood School records a “Withdraw” on school transcripts when students drop their [yearlong] credit courses. In all cases, if students drop courses, tuition for the Summer Session is non-refundable."