The Bath School disaster is the name given to not one but three bombings in Bath Township, Michigan, USA, on May 18, 1927, which killed 45 people and injured 58. Most of the victims were children in second to sixth grades attending the Bath Consolidated School. Their deaths constitute the deadliest act of mass murder in a school in U.S. history. The perpetrator was school board member Andrew Kehoe, who was upset by a property tax that had been levied to fund the construction of the school building.
Kehoe used a detonator to ignite dynamite and hundreds of pounds of pyrotol which he had secretly planted inside the school over the course of many months.
Hat tip to Dad.