It would be very easy when adding participants with the option Hierarchy synchronisation would offer the possibility to include the sub hierarchie(s) , so you can make a choise to ad participants from hierarchie X or one of the sub hierarchies form X, or ad participants from hierarchie X including all the sub hierarchie(s). So not only add the students from the sub hierarchies but the sub hierarchies to, so the groups are made from hierarchie X and all of the subs to.
Now, when i ad participants and want to make groups with the hierarchie synchronisation, and want to ad all sub hierarchies to make groups it can be a very time consuming job.