If you need to delete a session, you need to make sure first that there are no children or staff registered for that session first.

To check if your session has registrations attached to it, click on the Attendance button in the lefthand navigation menu. Click on the Session that you are trying to delete. You will be brought to the Attendance page:
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First, check if there are any contacts listed next to the Staff section.

If there is a Staff member listed here, then they have a Teacher Registration form that will need to be deleted.

To delete this Teacher Registration, click inside of the Jump to contact bar and search for the name of the contact in the Staff list:

Once you click on the contact name, click on the Teacher Registrations tab to be brought to a list of all the sessions the teacher is registered for:
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Click on the registration for the active session that you'd like to remove, and then click the Delete button in the top-right:
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Repeat this with any other staff members listed in your active session.

Once you are finished removing the staff registrations, go back to the active session in the Attendance module and click on the Roster tab:

Any children listed in the Roster will need to have their registrations deleted as well. Follow the steps listed above for the Teacher Registrations, but click on the "Child Registrations" or "Registrations" tab and delete the registration form for the active session that you want to remove.

Once the Teacher and Child registrations have been deleted, you can then go back to the Session and delete the session form result.