Over the years, I've enjoyed a number of flights with trainee FAs. IME the women were given a dress code that said solid-color pastel dresses, stockings, and heels (PanAm TV show?) and classic (rather conservative) business suits for men. One flight had almost twenty of the trainees on a narrow body, IIRC a KLM 737; they must have been observers. They all are assigned regular seats because apparently only official trained FAs can take the jump seats. I find it fun to watch the youthful eagerness and enthusiasm, but perhaps others object to the training being inflicted on paying passengers. It can slow the drinks service as well as meals, but I like to encourage then nevertheless as they all are on their best behavior throughout the flight.