And yes, it did happen with 7 other observers.. not a classroom though.
On May 21, the screwdriver slipped, the two hemispheres touched and created a burst of hard radiation. Slotin's instant reaction was to separate the masses by hand, by flipping the upper one to the floor. While he succeeded in ending the critical reaction and shielding seven other observers in the room, he exposed himself to a lethal dose (around 2100 rems, or 21 sv) of neutron and gamma radiation, in history's second criticality accident. Slotin experienced an intense burning sensation in his left hand. As soon as Slotin left the building, he vomited, a common reaction from exposure to extremely intense ionizing radiation.
Slotin's colleagues rushed him to hospital but Slotin was aware of his condition and, realizing he would die, is said to have remarked: "You'll be OK, but I think I'm done for." His parents were informed and a number of volunteers wanted to donate blood but the efforts proved futile. The accident ended all hands-on assembly work at Los Alamos. The incident was at first classified.
Louis Slotin died nine days later on May 30, in the presence of his parents. Two of the other observers also died a few years later with symptoms of radiation poisoning.