This is a term coined to identify things like jiggling breasts, cleavage shots, panty shots, bra shots, closeup shots of women's breast, legs, butt, etc., and mild nudity all for the purposes of "servicing" the audience and having no relevance to the plot or characters of a series. The term has a second meaning covering things like massive action sequences, explosions, fights, etc., but it mostly refers to the first meaning.
Self-explanatory. High school (grades 10-12) should play a major role in the title. Azumanga Daioh is an excellent example of this category. A typical High school series will include a Sports Festival, a Cultural Festival, and a school Trip.
