I wouldn't push it on the principal. They don't really have much power to inforce a teachers classroom behavior. It's certainly your right to request their presence but it wouldn't be guaranteed they could attend anyway. My son has been getting IEP's for years and for almost as long I have called a quarterly meeting with EVERYONE involved with him, in what I call his "support staff". So that would include his teachers, aids that worked with him regularly, therapists, psr workers, service coordinators, etc. I always hold this meeting at the school and as many people that were able to attended. I love these meetings because it really helps me see where my son is at, it lets each of his staff see what the other is working on with him and even lets them coordinate a bit. It gives us a heads up if there is an item on the IEP that needs to be reevaluated. There have been times where the principal was there and other times he/she wasn't.
If your son's plan really isn't being followed, you have to right to insist that it is. You may need to find an advocate for your son to make sure it's followed. Several years ago I upset the secretary at my sons school before it had even started for the season and she made things really difficult for me the rest of the year bu tit had to be done. My sons IEP specifically stated that he was to be in p.m. K and we had written it months previously. She called to tell me that there was no room in p.m. K and he had to be in a.m. K. She was very snippy with me when I reminded her the school was required to follow it and she very pointedly reminded me that it wasn't fair to the families of the two students she would have to drop from the p.m. class to make room for a special needs child. Well, if she had done her job properly in the first place, then it wouldn't have happened. But if I hadn't stepped forward and made sure he got what his IEP stated, then my son wouldn't not have gotten the best start possible.
Obviously, I have no idea what your IEP states, or requires. When you call your meeting, have the teacher prepare a progress report showing you where your son is at with his goals and if he's showing progress or even meeting/exceeding the goals. If he's showing actual progress, I really wouldn't worry too much about the teacher. But, if this far in, he's showing little to no progress then you certainly need to look into the teachers methods as well as reevaluate his IEP; the goals may be too high. When you have your meeting, document before what you expect to accomplish and so you can keep yourself on track. Document everything that has given you cause for concern and make sure it's all resolved to YOUR satisfaction.