To me the importance of music in the curriculum is in inspiring everybody in the school/country/world to enjoy and appreciate music. So the emphasis really should be on KS2 and KS3.
I'm no hugely talented musician, but I spent much of my youth in a variety of musical activities: choirs, orchestras, singing in musicals, a band, various instrumental lessons/grade exams and did O Level music as an extra. (It was very easy even then! We did it in 4th form=yr10, by just having one lesson a week at lunchtime for one year!)
At my boys' (state) school (performing arts specialism) they have two' lessons a week of music in yr7, one is just singing. They have inter-house singing competitions, 4 thriving choirs, including an all-boys choir and an auditioning chamber choir, superb concerts, Saturday music centre, etc. Last year they had a session in the hall where all the yr8 boys sang in their form groups, as an inter-form competition!
They also have instrumental lessons, orchestras, bands, la la la...
They offer an extra-curricular fast-track music GCSE. I think they do 1.5 hrs a week after school on Thursdays and complete it in 1 year.