We had something similar to raise funds for our school trip, we went to several stores and talked to managers and they were very happy to pay to have an add posted... Our adds were $50 each and that's nothing for most stores.
How Do I Write A Letter Soliciting Local Businesses To Advertise/donate In My Daughter's School Yearbook?
I would not write it at all, I would call, or go in in person, and bring your daughter too! That will make more of an impression that another thing to read will! In highschool I soicited ads for our newspaper staff so I am just telling you what worked best for me. And don't make it sound too pitchy or they will say no in their minds before you ever finish your shpeal. Put it like they are doing you and your daughter such a big favor, and how much they will be helping out and that is more likely to get their business!