Someone at the school should write some secular, school-spirit-based lyrics set to the tune of How Great Thou Art. Then the band could innocently insist that they just want to support the team by playing the song. "If people in the stands insist on singing different lyrics, that is not the band's fault."
Lots of different songs have similar tunes. John Fogerty once prevailed in a “he stole my song” lawsuit by demonstrating as much in a courtroom, with his guitar.