Yes. The rule is, every member of the chorus or orchestra may miss two rehearsals without an excused absence. Often, it is simply inescapable— everyone gets sick, and sometimes people have an appointment which cannot be rescheduled. However, you will of course be expected to make up the work missed. Members who miss more than the allotted number of rehearsals will be asked to leave permanently unless they personally contact the conductor and explain the specific reason for their absence. In this case, members will be asked to make up all the work and be able to prove they have done so.
In chorus, they will be required to sing in front of the conductor. For the first two missed rehearsals, the members of the chorus do not need to give any special notice; this is only necessary if, due to valid reasons, a third rehearsal must be missed.
In the orchestra, the situation is different. There, an expected absence should be reported by email to the conductor, as an absence may influence the rehearsal plan. The earlier this email is sent, the better. If absolutely necessary, a last-minute email will suffice.
For more information about this subject, you can read the “Attendance in Rehearsal” paragraph in this website’s “About Us” section.