With a reputation for thrift, Andrew Kehoe was elected treasurer of the Bath Consolidated School board in 1924. While on the board, Kehoe fought for lower taxes and was often at cross purposes with other board members, voting against them and calling for adjournment when he didn’t get his way. He repeatedly accused superintendent Emory Huyck of financial mismanagement. While on the school board, Kehoe was appointed as the Bath Township Clerk in 1925 for a short period. In the spring 1926 election, he was defeated for the position, and was angered by his public defeat. His neighbor Ellsworth thought Kehoe started planning his “murderous revenge” against the community at that time. Another neighbor, A. McMullen, noticed that Kehoe stopped working altogether on his farm in his last year, and thought he might be planning suicide. During these years, Nellie Kehoe was chronically ill with tuberculosis, and had frequent hospital stays – at the time there was no effective treatment or cure for the disease. By the time of the Bath School disaster, Kehoe had ceased making mortgage and homeowner’s insurance payments. The mortgage lender had begun foreclosure proceedings against the farm.

The Bath School disaster is the name given to a series of explosions perpetrated by Andrew Kehoe on May 18, 1927, in Bath Township, Michigan, which killed 45 people (including Kehoe himself) and injured at least 58. Most of the victims were children in the second to sixth grades (7–12 years of age) attending the Bath Consolidated School. Their deaths constitute the deadliest act of mass murder in a school in U.S. history. Between May 16, when she returned home from a hospital stay, and the morning of May 18, Kehoe killed his wife. He moved her body to a farm building before setting off incendiary explosions in their house and farm buildings. About the same time, he had arranged timed explosions in the new school building. The materials in the north wing exploded as planned, killing many students and some adults inside. Kehoe had set a timed detonator to ignite dynamite and hundreds of pounds of pyrotol at the school, which he had secretly bought and planted in the basement of both wings over the course of many months. The second 500 pounds (230 kg) of explosives in the south wing did not detonate, so that part of the school was not destroyed.

As rescuers started gathering at the school, Andrew Kehoe drove up and stopped his truck. During a struggle with Superintendent Huyck, Kehoe detonated dynamite stored inside his shrapnel-filled truck, killing himself and Huyck, as well as killing and injuring several others (among them a boy who had survived the initial bombing). During the rescue efforts, searchers discovered the additional 500 pounds (230 kg) of unexploded dynamite and pyrotol planted throughout the basement of the school’s south wing. These explosives, connected to an alarm clock that was supposed to act as the detonator, had been set for the same time as the other explosion. After the bombings, investigators found a wooden sign wired to the farm’s fence with Kehoe’s last message, “Criminals are made, not born”, stenciled on it. When investigators were done taking an inventory of the Kehoes’ estate, they estimated that, prior to its destruction, sale of the unused equipment and materials on the farm would have yielded enough money to pay off the Kehoes’ mortgage. One of Andrew Kehoe’s sisters claimed his remains and arranged for burial without ceremony in an unmarked grave at Mount Rest Cemetery in St. Johns, Michigan. The Price family claimed Nellie’s remains and had her body buried in Lansing, under her maiden name.