If the homework is stressing and upsetting her, don't force it. If she's just lazy and believes the bullshit the others are spouting about brilliant results without effort, then she needs a conversation about her future plans and prospects, and about the fact that working is as much a habit as lazing is.
Does she want to go to uni? Is she aware how competitive it is to get into the good ones, now half the kids in the country apply? Work is a habit, and while 2 hrs a night is not only bonkers, it's more than the advised average, she should be doing an hour a night and then at least 2 or 3 on Sunday, IMO, when she's nicely rested and fresh.
The "no work and still able to get good results" will partly be bravado and partly true. Some kids can do that, as some kids are just naturally adept at school work. Those are the kids who hit a wall at university when they can't rely on that approach anymore if they want to do well. Your DD shouldn't be forced, but she should be encouraged to understand the lifelong effects choices now can have. It isn't the brightest and best who do brilliantly at uni. Its the brightest and best who love what they are studying and also understand they have to work through the boredom wall to get on and to the good part.
There's also the possibility that she isn't suited to grammar school at all, and that she's not trying because if she doesn't try, then she hasn't got to find out if she's actually good enough. Kids constantly told they are bright can be afraid of failing, and prefer to be seen as brilliant but lazy, than dutifully plodding but just not top mark material. You may find she'd do a lot better if she was told that there isn't really any such thing as clever - the people who know a lot are the people who read a lot, and the people who excel in any area do so because they enjoy it and so put in a lot of hours. And also that everyone has to do that, and to break through the boredom sometimes, to do well.
She also needs sleep and down time. If she gets in at say 4.30, she needs time to rest, time to eat, time to wash and help out with a few simple tasks (lay table, put a load of laundry on) as well as time to study. Given she should take a 20 minute break for every 40 studying, and should be in bed by say 10 or 10.30, 2 hours working in the week may be ambitious.