There have been reports of CNX being difficult. Maybe they're true, maybe they're not but ultimately we made the decision to not worry about it and it was cool. We've flown in and out a few times. Got shouted at once but IO was prob having a bad day that time.
You can do as you wish. You paid for those classes as did loads of people so the Muay Thai School and Thai Govt did well out of it. The classes are probably overbooked so they won't miss you for 3 months.