As a former sales rep, I can tell you that when you told the receptionist your business, and didn't talk to the superintendent in person, you screwed-up. You needed to sell yourself to the superintendent, not the receptionist, and you can't overcome an objection by remote control. You should have made an appointment to meet personally with the superintendent, then sold yourself. At that point, if he/she raised objections, you could have overcome them. As far as overcoming a "liability" issue, there are two different ways to handle that.
1) If the concern is that you could get injured on school property and sue the schools, agree to sign a waiver of liability form for them,
2) If the concern is that you would create a trip hazard that someone else could get injured with and sue the school, you can agree to use only low impact methods or only recover surface finds, so no hazard would be created.
Now that the superintendent has made a decision on the matter, getting him/her to meet with you and discuss the decision will be near impossible. If you do somehow get to meet with the superintendent, be prepared to overcome the liability objection, as well as others, because no executive likes to change a decision once they have made one--he/she will likely throw out other objections to support the "NO" decision already made.
GL