Hi Op,
First of all what a bloody palava!
That being said I can only reiterate what (the more sensible, and dare I say it worldly) posters have said.
Going to the police may well end result in your daughter a criminal record, (remember you won't have any say in whether they charge), and even a caution hangs around on your record for years. The cops aren't here to do your parenting for you!
As previously stated a criminal record can make it vastly harder to get a job and bar her from certain professions. And of course stop her from travelling to certain countries.
What you need to think about is how you would feel two, or four years down the line if you have a teen who is still feeling the affects of being criminalised at such a young age. Do you want her stuck at home, unemployed (and potentially unemployable), or out in the world enjoying herself and developing as a person, this incident no more than a slightly embarrassing memory.
Remember "nos" canisters have legitimate uses and even their misuse isn't "that dangerous". I remember when "baloons" used to be sold by shot girls at various bars and clubs.
I'd give her the bolocking of her life, dispose of the canisters, (in a public bin) and find out where she brought them- and once equipped with this information put in a cheeky call to trading standards so other young folk don't end up buying them.