It's a little more than a pop; the bottle builds up quite a bit of pressure before it ruptures - I know this because we use to make these quite a bit when I was a kid. That said, are they dangerous? Potentially. Does this merit the reaction taken by the school and prosecution? Hell no.
AFAICT, it's not a deflagration, it's not a detonation, it's not an "explosion" in any technical sense.
It's a loud noise produced the same way as a popping balloon, it's just at a higher pressure. Plastic bottles are designed to contain the maximum amount of pressure for the amount of plastic they consume, so they don't crack and fizz out like a punctured soda can. They fail all at once.
So... it turns out this has to do with Hydrochloric Acid being considered a Pesticide by the EPA since it can kill bacteria. This means they have to register its label for review and feedback with the EPA, as do companies that make bleach and a wide variety of other things.
The "inconsistent with its labeling" part deals with using it as a pesticide.
If you are found guilty (consumers have never been charged under this that I can find a reference to) as a non-commercial user, the penalty for the first time is a warning letter. According to the controlling Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), fines can only be administered if you violate usage another time after receiving a written warning from the EPA.
And none of this has anything whatsoever to do with what she is being charged with. She is not being charged under the Insecticide, Fungicide, and Rodenticide Act.
Those concerned that the FIFRA means you must strictly limit use of EPA controlled pesticide products to specified uses only should consider the fact that since ordinary tissue paper is considered to control the spread of viruses when you have a cold, it is regulated as a pesticide as well under the EPA.
Notice this one allowing an exception - Kleenex will be allowed to remove the ordinarily required "Keep out of reach of children" warning for their EPA regulated pesticide known as Kleenex brand tissue paper.
According to the theory that this act strictly controls usage, using Kleenex to clean your glasses is a violation of federal law as that usage is not listed on the label.
In some other cases, "the penalty for violating labeling rules is death and the punishment may be carried out at the time of offense without benefit of a trial as a natural consequence of the laws of nature and chemistry."
Here's the MSDS data sheet for "The Works" http://www.chemtura.com/msd/external/e/result/report.jsp?P_L...
IIRC, that combination should release H2 and a little heat, but not enough heat to ignite anything. So the bottle basically went "fizz" and "pop".
Some of those plastic bottles can make a loud pop.