Hi there
I work with this age group and I do sympathise with you... it is so hard to motivate them to study and develop good study habits as they are still so young and just do not see how it is relevant to their futures, even though it really is!
Firstly, I would consider possible reasons why she might be less motivated to study. Does she seem down or anxious about anything? And have other activities or socialising taken a back seat?
You said her focus is not great - could she possibly have underlying SEN such as dyspraxia, inattentive ADHD or or sensory processing issues?
It is interesting that she actually told you she didn't study. Do you think she might have been looking for a reaction from you? Was she possibly trying to open up a conversation there?
The book "How to talk so teens will listen and listen so teens will talk" is a classic and has some great practical examples of ways to engage with teenagers and have productive conversations.
A lot of my students struggle to link what they are studying now to how their lives might be in the future... I find that when teachers talk to Year 7/8s about GCSE and A level results, chances of going to university and career choices etc they tend to switch off as those things are still a million years away in their heads. Instead, try to encourage her to learn for the sake of learning and get her enthusiastic about just 'knowing things' and being capable as she is clearly a bright girl.
I think it's really important NOT to to nag about studying and never let it seem like a chore, but do acknowledge that not all tasks will be completely riveting - sometimes we just have to do them to get to the more rewarding and exciting stuff later on. Let her see you enjoying learning and try to encourage her to join you, even if it is just enjoying a documentary, playing along with quiz show on TV or sharing interesting things with her that you have read in the newspaper.
Could you do some activities over the holidays which might spark an interest in a particular topic even if it's not directly related to the curriculum - such as a a trip to a museum, historic place or some fun science experiments? (Loads of simple ones on YouTube which are fascinating and easy to do at home, making slime for example).
Praise her for her efforts, even if they seem relatively small compared to how things were in Sept. It sounds like she's slipped back a fair but so it might be baby steps for a while but this should all be encouraged - all kids love praise and recognition.
Good luck!!