I remember this being a radio 4 book at bedtime, probably at least 15 years ago. It could be as long as 25 years but I don't think so. The main character is a teenage girl, no mother and father is a teacher/lecturer. She is planning increasingly weird ways of killing her father's new girlfriend I think. She comes across as very arrogant and delusional - believes she will get top a level grades with no effort and is anticipating an Oxbridge place. I think the ending is that she reveals she has been diagnosed with anorexia and realises that she had lost all contact with reality. Then her new stepmother turns out to be plotting her death.
I may have some of this wrong.
It feels quite Barbara Vine but I think I'd have remembered that? Possibly Patricia Highsmith?
It has been annoying me for years and I would be so grateful if anyone recognises this! TIA