[Retric]

I can't find the school example, but this stuff is a hodge podge of different rules. The basic problem is:

Depending on who "owns" the waste, its handling and disposal is regulated differently. " In 10 C.F.R. § 20.2002, the NRC reserves the right to grant a free release of radioactive waste. The overall activity of such a disposal cannot exceed 1 mrem/yr and the NRC regards requests on a case-by-case basis. Low-level waste passing such strict regulations is then disposed of in a landfill with other garbage. Items allowed to be diposed of in this way are: glow-in-the-dark watches (radium) and smoke detectors (americium) among other things."

https://en.wikipedia.org/wiki/Low-level_waste

"Low-level waste (LLW) is nuclear waste that does not fit into the categorical definitions for intermediate-level waste (ILW), high-level waste (HLW), spent nuclear fuel (SNF), transuranic waste (TRU), or certain byproduct materials known as 11e(2) wastes, such as uranium mill tailings. In essence, it is a definition by exclusion, and LLW is that category of radioactive wastes that do not fit into the other categories. "

Of note, there is no exclusion for naturally occurring radioactivity.