You can use Attendance Patterns to automatically mark specific sessions with a chosen attendance code when someone is expected to be absent regularly. To do this, go to Office > Registration > Attendance Patterns. You can set these patterns for individual students, marking them as X, D, or B for either morning (AM) or afternoon (PM) sessions, Monday through Friday.
Using Multi-Select fills all listed students with the chosen code for that session. You can set Attendance Patterns starting from the upcoming week until the end of each half-term in the school year. Once you've chosen the patterns, click "Set Attendance Pattern" to apply it from the displayed date.