I read a short story some time in the mid-2000s; I believe it was part of a collection of stories published in one volume and not all by the same author.
The viewpoint character's grandmother has certain house rules that must be obeyed; the strangest of these is that nobody is ever allowed to leave a tap dripping. On one occasion she completely freaks out when the viewpoint character leaves a tap dripping just to see what would happen.
Eventually we discover why she has this irrational fear (I can't remember how - possibly via flashbacks and a change of viewpoint, or perhaps her grandchild discovers his diary or something). When she was a young girl, her class in school was going somewhere on a trip, but she couldn't go because her family was too poor. The next day she came into school and saw all her classmates sitting in class, but when she spoke to them, the teacher acted as if she was making a sick joke. Eventually one of them tells her what happened:
"There was an accident while we were driving. The school bus went off the road and into the river. And none of us could get out."
(quoting from memory)
Then the entire class turns transparent and begins to drip, drip, drip onto the floor.
The story ends with
the sound of a dripping tap and the grandmother being found dead in her living room.
It was a nice chilling tale. Any idea what the title and author might be?