Andrew,
This should be entered via course manager (tools>academic management). Usually students should be assigned to the course automatically through enrolment in the relevant class however lots of places this can go wrong (I am assuming they are allocated to a specific class for maths resit?). If they are in a class sometimes the level in course manager is wrong as the timetable export has all the KS5 courses linked as say A-levels where this is a GCSE. The course needs a QN (qualification number) as well.