School Days by Robert B. Parker

Lily Ellsworth - erect, firm, white-haired and stylish - is the grand dame of Dowling, Massachusetts, and possesses an iron will and a bottomless purse. When she hires Spenser to investigate her grandson Jared Clark's alleged involvement in a school shooting, Spenser is led into an investigation that grows more harrowing at every turn. Though seven people were killed in cold blood, and despite being named as a co-conspirator by the other shooter, Mrs. Ellsworth is convinced of her grandson's innocence.