This is a pretty familiar scenario
What worked for me (and still does for checkouts) is ...
- Decide which instructor seems to gel with your learning style
- Book your sessions well in advance
- Politely but firmly book your session with the words "I'd like to book a lesson with Ellie Vator, please, in G-AFFE (always good to use a familiar steed to begin with)"
- When they say "I'm sorry she/he's booked/sick/holiday/injail" be a bit flexible on dates/times and ask politely "When will she/he be available)?"
You are the customer and it's quite reasonable for you to exert a little control over how your (presumably) hard earned five grand is spent.
No-one has ever taken offence when I have used this approach.
Have fun!
SD