Try 453.000, 452.050, and 462.275. They are all repeater frequencies licensed to Wylie ISD. Currently, it looks like they are unidentified in the database.
Another thing to check (you would know better than I) is the business frequencies if the school buses are run under a contract. Here in Amarillo, the buses belong to a private concern and although they have radios, the license is under the contractor's name and is an IG type license.