If your school has been with school-links for a while you may have some auto groups. These pre-date the dynamic grouping function.
Auto groups are only automatic in their initial creation: the group members remain the same over time. Auto groups do not automatically update to reflect new users who fit the criteria, or those leaving. The dynamic grouping function allows you to send to all users who fit the criteria when your message is sent.
We recommend you create dynamic groups to replace any existing auto groups.