Erring on the side of caution here would mean sending the student home for the day with his plate, and a note that says "student was sent home because we're unc

Erring on the side of caution here would mean sending the student home for the day with his plate, and a note that says "student was sent home because we're uncomfortable with any level of radioactive materials in the school. They're not in trouble, but please don't do it again". It's still more than is really necessary, but I can understand not wanting to expose other people's children to any level of unnecessary radiation. There's a huge difference between "a thing designed to eat out of, with radioactivity as a trinket" and "a thing literally designed to cause cell death". Even in the era when radiation was marketed as healthy, we weren't giving people doses anywhere close to the output from that radiotherapy source. That thing was outputting almost 5 Grays per hour at a meter away. You would have a high enough dose for Acute Radiation Syndrome after 12 minutes of exposure at a meter away. A little over an hour of exposure puts you in the "probably going to die even if you get to a hospital" range, and 6 hours puts you in the "there's nothing we can do for you, you'll be dead in less than 48 hours" range. It's just nonsensical for a consumer product, even in ye olden days. People would have noticed that anyone who bought the glassware almost immediately became seriously ill with mysterious symptoms. There are lots of products from then that had more radiation than was healthy, but it was in the "long term exposure is going to cause cancer or other diseases" form, not the "you are going to die imminently" form. Even in the Radium Girls, who were ingesting radioactive paint, it took 6 years from the opening of the factory to the first recorded death. They also had an unusual method of exposure; I'm willing to bet nobody at the school was debating eating the plate. The student would have been dead or severely ill if it were anywhere close to Goiania. Acute Radiation Syndrome has a fast onset; the longest time between exposure and symptoms listed is 6 hours. The fact that the student appeared to be healthy should tell you that the dose isn't immediately dangerous. That it sat in an antique store (pulled from another article) without the owner falling over dead should be another clue. Further, this was just a quarter sized shard of a plate, so the owner presumably has several. If one of these shards emitted a dangerous level of radiation, the whole pile in the antique store would have poisoned an entire neighborhood. The student also brought a Geiger counter, so they don't even have to guess at what the dose was; they could measure it and go from there.