Yes, definitely buy a carré neige
(or a carte neige).
The Carré neige is a short term insurance that will cover all costs associated with a search and rescue operation and a medical evacuation. Some medical expenses are also covered, but to a small amount. It's not a medical insurance.
You can buy it with your pass. Just tell the cashier you want a pass "avec assurance
". It will cost you around 2 or 2.5 E / day. No paperwork, your pass is the proof of your insurance.
Actualy, even if you had another insurance, I would recommend a Carré neige, as it is ubiquitous in France and can spare you long explanations in a foreign language while in a stretcher...heaven forbid.
The Carte neige has similar benefits, but is typicaly purchased for the season, by frequent skiers. It's frequently associated with a license from the French Ski Federation. The daily rate is cheaper, but you wont need it.
Now, I don't know if you're (officialy) going to France as an instructor or as an "amateur" : The Carré Neige does not cover you as a professional.