my little sister is a complete nightmare shes nearly 6, shes very bright and she got bored at school and spoke to teachers and they gave her harder work and this seemed to help as she had more to concentrate on.
also does she go to any after school clubs? i know some are expensive but something like st john ambulance badgers is only a couple a quid each week, they have to work towards different badges and get a uniform, i was in from when i was 6 untill 17. does give you a sence of achievement.
also well done for sticking to the punishments but just make sure her achievements dont go un noticed, even the simple things like sitting still on a chair eating dinner ect tell her well done and thank you for being good, a cuddle and a smile, it will take time but she will relise how much nicer it is when shes not being punished.
Also talk to your partner make sure you areboth on the same page with how to deal with the situation.
Good luck with it all