I'm doing AS music this year- also play piano and flute

The standard level of performance at AS is c.Grade 5, and c.Grade 6 for A2.
Try looking on
http://www.abrsmpub.co.uk for pieces that aren't on the grade 5 or 6 list (i was told the examiners get bored of listening to these pieces over and over- plus, if you do something different, it will make you stand out from all of the others

).
Suggestions for the piano are:
'Danza de la rosa' by Granados from 'A romantic sketchbook for the piano Bk 5- (which is around grade 6 standard)
'Le Petit Negre' by Debussy from Bk4 of the romantic sketchbook (around grade 5 standard)
Any of the 'Kinderscenen' by Schumann (between grades 3 and 6)
Some of the 'Spectrum' pieces are also nice- bk 3 is grade 3-7 standard.
Suggestions for the flute are:
'Sicilienne' by Faure- It's on the grade 5 list and is a very nice piece.
The 4 pieces from 'The Elements' by Robert Hinchliffe are also very nice- and around grade 5/6 standard.
good luck with your GCSE music btw