Why can't you just say exactly as it is? I could never understand this need to wrap what people want to say in a pretty cover or else they are afraid to come across as rude or whatever. You are objectively pointing out to some fact that takes place. There is nothing personal about it. You are not petty. You have the right to do that because you pay for a certain amount of time and are entitled to get it. You are not accusing the T of anything horrible, not making a big deal out of this. No reasonable person would take offense if you let them know that they've been habitually late and that it takes away from your session time and that you'd like to have all the session time you are supposed to have. If they take it personally, it's their problem, not yours.