BPR40's are programmable, so they may be customized to a user's assigned frequencies. Unfortunately unless they are transmitting frequently, finding the frequency using a HT's scan function is like finding a needle in a haystack.. I use a Radio Shack Pro-83 to find unknown frequencies, but with a little patience and deductive reasoning, you might still be able to find it. Try doing the following to narrow down your search.
1. What is the length of the antenna?
-As far as I know...BPR40's come in 2 flavors...UHF and VHF. The UHF ones have a shorter antenna, the VHF ones have a longer antenna. If you can put eyes on the radio, you might be able to at least figure out what band it's on and search from there. VHF business band and public safety users (depending on how the school is licensed) are most commonly in the 150-156 range on VHF, and 453-456, 460-467 on UHF.
2. Check the FCC database.
-The Universal Licensing System (ULS) on the FCC database is a wealth of information. Think of it as the "raw" version of RadioReference. You might not be able to find individual frequency use, but once you have the list of frequencies licensed for a given licensee, you can sleuth it out yourself.
The link to it is: FCC Universal Licensing System (ULS): ULS Home
3. Try checking the "DOT" or itinerant frequencies for each band. Some of these are licensed, some of them don't require a license, and often users may be using a frequency that requires a license, but due to them running low power and nobody filing a complaint, have gone unnoticed by the FCC.
Check these references for specific frequencies:
Business band - Wikipedia, the free encyclopedia
The Microwave Rider - Garbage Channel Scanning Article
I remember there is also a link on here for itinerant and DOT frequencies, but I cannot locate it at the moment.
Good luck, and feel free to private message me if you have any questions you don't feel like posting to the list.