No such thing... Proper ragu ingredients vary a lot too!
That's certainly true. The recipe I use has chicken livers, milk and nutmeg in it.
As for the school in the OPs question, it would seem like a sensible compromise to enable a bunch of kids to learn to cook a meal in a limited period of time.
I like my recipes to be written like a haynes manual- I want times, temperatures, proper amounts, info on what to do if it goes wrong (or in fact just info on what it looks like if it goes wrong).
Sadly recipes are only and can only ever be a guide. Food isn't consistant in the way that a car engine is so detailed instructions don't actually work very well; you have to rely on your own knowledge skill, and palate.