Ghost Hunt

This is an interesting series about a group of high school studnets who investigate hauntings. There are some adults helping them. One good thing the series does is bring up alternative explanations for phenomena first; if that doesn't explain everything, then they get to the actual ghosts and spirits.

Some of the manga, in synopsis

Volumes 1 and 2
Volumes 3 and 4
Volumes 5 and 6
Volumes 7 and 8
Volumes 9 and 10