Time of the Ghost. The scary bit with the ghost of a long forgotten king in the barrows does really haunt me.

eeee 😊 i hope it's not a red herring! i can't have read this in the last twenty years so am a bit vague. the sisters are very distinctively drawn & they live at a school IIRC and there is a mystery? Hopefully more DWJ is never a bad thing even if it's not this one!

It was it! I’ve just read it and my skin crawled at realising it was the one. Absolutely marvellous.