The concept is, to select Yr 10 Music, the student must have completed Yr9 Music with a C+ or greater

1.  On the Subjects Page, create new subjects for any option that will have a grade prerequisite ie Yr 9 Music C+  (highly recommend giving the subject a meaningful name which includes the eligibility grade)
You are going to assign these subjects as “completed subjects” for any student that has received C+ or greater in Yr 9 Music  
Then create a Subject Rule|Prerequisite “to select Yr 10 Music, you must have completed Yr 9 Music C+”

2.  On the Manage Data page, click “Download XLSX template”, open the template and at the bottom of the template select the Completed Subjects tab

3.  On the Completed Subjects tab, enter all the Student Codes and the Option codes for all the dummy subjects for the students who have received the relevant grade.  
ie you might have 10 students who have receive a C or higher in Yr 9 Music.  Therefore the Completed Subjects tab should include 10 students all with the Yr9 Music C+ option code

Repeat this process for any subject that you require a certain grade to be eligible to select, then save the template

4.  On the Manage Data page, assuming all students are already loaded in Web Preferences, click Import XLSX template.  This will add the dummy subjects as a completed subject for the relevant students.

Note:  the new subjects you create, such as Yr 9 Music C+ will not be enabled and will not need to be in a Preference Group.  They are only required to be assigned to relevant students as completed subjects and then used in a Subject Rule|Prerequisite.