The Bath School Disaster

From Wikipedia:

The Bath School disaster is the name given to 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. He blamed the additional tax for financial hardships which led to foreclosure proceedings against his farm. These events apparently provoked Kehoe to plan his attack.

(Thanks to Girard31 from the comments for this)

Related:
The Bath School Disaster page.