You may be on the Sentry staff (as a reporter, photographer, or artist) without taking the class. All grade levels may join the staff.
There are also two Sentry classes. It is NOT necessary to have prior experience to join the class, although experienced students will be given priority for the Sentry II class. If you are trying to sign up for the Sentry I or Sentry II CLASS, go back to jpsentry.net/apply and choose that link.