Do some of your Sixth Form perhaps help organise/ run events, do any of them assist in Arts lessons at KS3, do they perhaps do an Arts activity as part of their Duke of Edinburgh Award, have you included backstage crew and asked Art, Drama, English Literature (I seem to recall that this counts at KS5?), Dance and Music staff for numbers of students who stay in their frees or after school. ... hope that helps you get the numbers you need!!!!! The form is total brain ache. Make sure you try and get data/ admin staff collating plenty of the stats for you too. Good luck.