Mostly VERY good advice.
But I'd add-- do not expect a lot of expertise re: low threshold food allergies from a local guide. If it's rare here at home, it's rare there, too, so expect about the same general level of awareness as from a random stranger HERE.
MedicAlert Canada has services that MedicAlert USA does not. Call and discuss MONTHS in advance if possible.
If you're sending a teen without a parent (which, by the way-- I'm not sure that I'd encourage with: a) very low threshold and/or b) allergy to egg, milk, or wheat, and/or c) really severe anaphylaxis history or atypical presentation of anaphylaxis), be sure to discuss a VERY VERY VERY detailed action plan in advance, and with your allergist.
You need a complete decision tree on most possibilities, and then you need to role play it.
You are on the airplane and wonder if you're getting sick... sure hope not, but your chest feels kind of 'tight' like that...