During the pandemic, HALO noted on its website that monitoring indoor air quality was an approved use for federal COVID relief money.
A leading provider, HALO Smart Sensors, sells 90% to 95% of its sensors to schools. The sensors don’t have cameras or record audio but can detect increases in noise in a school bathroom and send a text alert to school officials, said Rick Cadiz, vice president of sales and marketing for IPVideo, the maker of the HALO sensors.
The sensors are marketed primarily for detecting vape smoke or THC but also can monitor for sounds such as gunshots or keywords indicating possible bullying.
I don't know that I'd trust a non-microphone sensor to be good enough to actually identify keywords, but you could probably tell if people were yelling or something.