While guns have been a common denominator in mass slayings at schools by teens, there’s another familiar element that seems increasingly to be minimized. Some 90 percent of school shootings over more than a decade have been linked to a widely prescribed type of antidepressant called selective serotonin reuptake inhibitors or SSRIs, according to British psychiatrist Dr. David Healy, a founder of RxISK.org, an independent website for researching and reporting on prescription drugs.