Each week, a student receives a score out of 20 for class participation. This is based on how well they are prepared on the music, and how well they are participating in the class/rehearsal.
Students also receive a score out 10 for each weekly lesson, based on the same criteria. With field trips, testing, teacher scheduling issues, and other conflicts, students are normally scheduled for 7-8 lessons each quarter. This means that if a student receives perfect scores for class participation but doesn't attend any lessons, the best possible score is around a 68. The reality is that when students miss lessons they are not as proficient on their instrument, and their grade goes lower.
This is tempered somewhat by the fact that performances raise students' grades. But, if a student is sick and misses the performance, that "bump" in the grade doesn't exist.
Worried about grades? Don't be. In 2017, the 4th quarter average score in the 7th & 8th Grade Band was 91.3, with two-thirds of students receiving a 94 or higher. The average grade in HS Band was 95.8. Do what you are supposed to, and you will get a good grade.
