It really is nothing to do with student ability and everything to do with money. A teacher-hour spent with 6 students is twice as expensive as a teacher hours with 12 or three times more than with 18. Simple as. The Head has a responsibility to make sure there is good value for money. My school allowed a 7 student GCSE to run. Now we are a little less well off it might have to be higher. It's always a minority subject. I'm quite impressed it appears in two option streams to be honest? Running GCSE after school is often not in the best interests of the music teacher. If I did this, in my very accademic school, I know that I would get every single student opting to do it after school so they could pack their time table with extra extra maths or quadruple science or a third language!