A new study finding a rise in suicidal thoughts and attempts among young people adds to the research pointing to a decline in mental health among U.S. children and adolescents.

The study showed the proportion of young people treated at 31 U.S. children’s hospitals for suicidal thoughts or attempts more than doubled between 2008 and 2015, from 0.66% of all visits to 1.82% of all visits. Rates were higher during the school year than in the summer, and nearly two-thirds of the visits involved girls, according to results published...