Typically this is an easy process. When I worked at HRC and Soldiers would contact us about these issues we would refer them to the schools branch (usually this was for TDY enroute on a PCS). The school branch would document the issue and then reassign the Soldier to another class date.
If you are TDY and return to your unit there should be an OML. If should be rather easy for your leadership to use the medical documentation to remove you from the OML and send the next person on the list. Just make sure they reschedule you as soon as possible.
