Totally disagree with counselling! That's an overreaction. She's being a typical 12 year old. It's the stage of life where teenagers are starting to break away from their parents and gain independence, its natural that the people she rebels against most are her parents, because they set the rules and boundaries, and it's those which she's rebelling against.
Grandparents and school is slightly different. Because at school it's the rules that everyone follows, so she's more likely to comply if her friends are. And as grandparents you may set the rules for in your home but it's not the same as her parents who (in her eyes) are in control of her life. It's totally normal behaviour. I work in a school and we get lots of kids like this, their parents are always shocked that they're lovely and polite to teachers but aren't at home.
She will grow out of it, but your daughter needs to make decisions into less of a power struggle, offer choices where she can (like you would a toddler really!). So some boundaries need to be firm and non-negotiable (like her curfew and pocket money) but other things may be more of a negotiation (like dinner, staying at friends houses, what chores she does).
Also, does she have much independence? Does she walk to school? Get to spend time in town or the park with friends at weekends?