Vanderbilt:
https://blair.vanderbilt.edu/departments/ma5.php
https://www.vanderbilt.edu/vuband/aboutsog.php
And students from Belmont University also participate in Vandy’s marching band:
http://www.belmont.edu/music/degrees/music_undergraduate/music_education.html
U of Miami is a little bigger than you want (around 10K undergrads), but still smaller than most schools with serious marching band programs. https://www.bandofthehour.org/
https://musiced.frost.miami.edu/degrees/bachelor-of-music-in-music-education-bm/index.html