Try searching for school district or bus company licenses on the FCC ULS page here. If you don't come up with any license results, they may be leasing repeaters and radios from a 3rd party vendor under FCC Part 90.35. You can also try entering the school's address on Antennafinder.com to see if any antennas exist on that site. If all else fails, visit the site with a close call enabled scanner and see if you get any hits (try to be as discreet as you can to avoid trouble)
all school buses will have their company name on each side of the bus. doesn't matter if the school is elementary, middle, high school, catholic school...watch for the buses, jot down the name and then refer to the link i sent.
i'd listen to the frequencies for those companies and see if you can deduce which company is serving which school...a little detective work. or drive by the schools in question aboit 15-20 min before afternoon dismissal and note which buses are where.