This should be done as part of the creating the next academic year structure and not in the current year.
Select Focus > School > Pastoral Structure > Next Academic Year Structure to display the Next Academic Year Pastoral Structure.
Expand the Pastoral Structure until you can see the Registration Group to be removed.
Right click on the name of the group to be removed and choose Remove from the menu. If the group contain members, a warning will display to move the members to another group. If so, select the destination group and click OK. If not, a different warning will display to delete the group.
Click Yes to continue. The old class is removed.