Hi Martin
It may be a combination of your Content and Access settings and what you consider to be a manager
Manager
Firstly to distinguish between being a manager of the students, in the sense that you are attached to their user profile, or a 'manager' in the sense tha you sit above them in the hierarchy. Check the students' details at:
Settings > Positions > Primary Position - make sure you are listed as their manager.
Content settings
Then you can create a report based on 'Show by the user' as Craig mentions.
Alternatively, if you don;t have that manager association in their profile you can base it on hierarchy positions;create a report based just on 'Show by the user's current organisation' or 'Show by the user's current position'
Access settings
Finally, Access may be affecting what you can or can't see. These setitngs relate to restricting/providing access by Totara Role, rather than hierarchy structures. If you have specified a role, either in system or any context and you do not have that particular role then it will not be visible. If unsure, try setting it at 'All users can view this report', the content settings should still take affect so that not everybody will see the report content.
Give that a try. I am around today if you want to have a skype call.
Andy