I agree with the first part of this - plan as though for a meeting at work, with written notes you can refer to and a definite idea of what needs to be covered. However, there's a huge difference between planning as though for a work meeting you're attending, and planning for one you're running, and trying to run a meeting that isn't yours to run can cause offence. For this reason I definitely disagree with the idea of turning up with powerpoint presentation. Unless it's prearranged with whoever is actually in charge of the meeting (the principal in this case, presumably) that's rude, and very off-putting. Now, preparing a powerpoint presentation just as a thought experiment in how you *would* give one *if* you were going to, maybe even using a printout of the slides as an aide memoir, that might be useful.
It sounds as though this is a case where the chief problem is that you have to effectively run the meeting, i.e. make sure it covers everything and doesn't get side-tracked, while not sitting in the seat of the person who should be running it, and not putting that person's nose out of joint by obviously doing their job... Not such an unfamiliar position for many people, I'm sure! Concretely, I think the no-brainer is to have a concise sheet of notes for your own reference, listing points to be covered and key things you want to remember to say, and actually sit there with a pencil ticking things off and checking everything's covered. You can even say, "I just want to make sure I don't get home and think "oh I should have said"". A more delicate issue is whether to have something in writing to hand out to other participants. I would tend not to unless there is concrete new information you want to make sure they all have. A compromise might be that if you have, for example, written a good letter to the principal that the others present may or may not have seen, you might want to take copies of that to hand out. That avoids doubt about whether you're happy for everyone to see it, and might serve as a reminder too.
Don't be afraid to be a bit broken-record. If you raise an important point and discussion veers off, raise it again; you can do it nicely, "I just want to be sure we're clear what the plan is about ..." or whatever.
Also, a good tactic for afterwards can be to send a letter thanking those present for the meeting, and summarising in the letter your understanding of what was agreed. (Unless the principal is going to do it - although in our case that led to a comical situation in which we thought the head had agreed to write and summarise, it didn't happen, so eventually we did so, and the result was that the head treated that as a trigger for another meeting!)
Good luck...
