I'm pretty effing terrible at leaving messages myself, and speech and communication is like, you know, my thing. I usually start off well, like:

Hello this is Laura _______ from _______ calling for Mrs. So and So. I need to set up a meeting with you, regarding your son Billy. Please call me as soon as possible at the school to set up the appointment. The number is...oh wait...I don't have the number memorized...it's programed in my cell phone, and I just do speed dial if I need to call the school myself. Uh, hang on, I know we have it written down here somewhere, uh....um....uh....I can't find it. But you probably know the school's number, so, uh, call me at the school at your earliest convenience, and uh, we'll set up a time to meet. Oh yeah, this is Laura _______ from ________ calling about a meeting for ________. Uh, thanks, bye.