If it was me, i would write out a list of questions/areas i am concerned about and ask them to respond in writing prior to your meeting. I would then take this to the meeting and use it as your basis for discussion. I am very avoidant of conflict, so this is probably the only way i would have the guts to do it. Also, it means you have written documentation you can use to take things further if required. They are also more likely to put the time and effort into responding as they know that you will be able to use it, rather than just blow you off with non-specific statement. Make sure your own questions are really specific, and take your time writing it. If you need to move the meeting, then do so, as its more important that you are prepared so you can actually get a resolution.

I would either email or give it to the teacher in a sealed envelope with a note saying something like "Dear x, Thanks for agreeing to a meeting with me regarding my children's progress. I am really pleased the principal is going to be joining us, as i would like to get a good understanding of how they are doing and what action we can take to ensure that they are doing the best they can. I have made a list of questions i would like to discuss in our meeting, i would really appreciate it if you could respond to these in writing prior to our meeting to make sure that i have everything clear in my head. I hope you don't mind this approach, i really want to make the most of this meeting and in the past i haven't come away with a good understanding or any clear outcomes."

Once you have your answers from them, get as many different opinions as you can. I am sure there are lots of teachers on here who could provide some advice, i would even consider speaking to another local schools principal about them to get their opinion.

Good luck!