Your district's attorneys are very well versed in school district law. That's it.
Unless you hire a specific attorney who is versed in copyright protection and copyright laws, your school attorneys cannot give you any better information than you can come up with yourself. Copyright is a very specialized branch of the law, since it is international in scope - and each country has its own laws to go with in addition to UN resolutions.
Cases in point: Copland, Shostakovich, and Sousa. Much of Shostakovich's (and Stravinsky, and other Russian composers') work came open in public domain in the 1980s, yet then when we passed the Mickey Mouse copyright extensions (from 50 years post-authorial-death to 75 years) in 1990 many of those PD pieces became retroactively no longer PD. It was also very routine in the Eastern bloc as well to completely ignore copyright on Western composers - and vice versa. Copland was routinely performed in Soviet areas because they blatantly stole his music because they "didn't recognize" our copyright.
But, with Sousa for example - SPECIFIC arrangements of his music remain under copyright because those copyrights are owned by publishers, NOT by Sousa. Arrangements of the original Sousa music are fair game, and the Marine Band is doing its Sousa project, providing fair use copyright-exempt versions of his music available for download. However, if you perform a copyrighted arrangement yet try to use one of their scores instead, you're in violation - a band was nearly DQ'd at UIL for this within the past couple years.
There's a TON of case law here, and a ton of stuff to dissect. Your best places to start, as you state, are the district policies - but I've seen district policies that are in direct violation of copyright because the administrators who wrote the policy didn't know the law. Check district policy, but also look into copyright.gov and its information on fair use.