Try asking the schools accountant, he or she should have a form you can use. They are very common, some stores such as Staple's require them and send you one to fill out. Maybe the sales rep could ask their accounting dept. to send you one to fill out.

Help! We are currently involved in a fundraiser with selling Candles in a Jar and the rep wants a copy of our Sales and Use Tax Permit!?!..so she won't have to charge us state sales tax..I've called the State Auditor--no help--, emailed the manager of the Sales/Use Tax section of Dept of Finance for our state--no help; how do i get one or who knows what I'm to do next?..any info will be more help than I've gotten, so far..