Well, several things could happen...
You say she's good at English and able (and a girl) - she might well get a top grade without putting much effort in & then assume A's are* given out like candy & so coast for everything else. So your objective isn't achieved and she gets an out for English lessons for the next two years, thus resulting in her doing even less English overall than she otherwise would have.
She's an able child & at a grammar school, so I assume she's capable of A*s and As for GCSE & should be aiming for that. If though she gets a lower grade, either with or without effort being put in, it might instead of making her pull her socks up make her feel that lower grades are acceptable after all. Once you've got one, then a few more won't matter type attitude - after all the world didn't just end. So it could easily not have the desired effect (as well as spoiling a potentially clean sheet of higher grades).
It might give her the kick up the arse that she needs - though why moreso than the media studies and ICT that she's already doing early? (ICT is supposed to be the hardest GCSE), but it could so easily backfire. It's probably just her age & hormones making her unsettled at the moment - if the school is league table conscious (which from what you've said it is) then I'd trust them to give her the pep-talks she needs to get the good grades.
That's my thinking anyway.