I really think you need to chat to teacher about this and explain what's going on. Maybe the teacher can stop saying it's ok to extend the work.
I've said elsewhere that I think kids learning to figure out what is required and then deliver what the teacher wants is an important skill. You can help with this. There isn't anything wrong with her doing deep study or even putting extra effort into projects but I feel this will set her up for bad habits when the workload takes off. Sounds like you have a wee perfectionist on your hands!
