Because Jr. Days are open to anyone to sign up. You don't have to be a FBS prospect to come to campus and see what it is like. All the people I know in coaching world have set up every Jr. Day for their athletes. I did this as a favor to Cooley because he asked. I am sure if he asked Calvin's coaches they'd be more than happy to help set it up. You can sign up for Jr. Day at every school in Tx/USA as long as you can find their Director of Recruiting or HS Relations guy on the school website and email with the athletes class, position, name, and school just like we list on our thread titles.
http://www.ncaa.org/sites/default/files/June2016_DIBowlSubdivisionFootballCalendar_20160613.pdf
http://www.sbnation.com/college-football-recruiting/2014/12/17/7401635/ncaa-football-recruiting-periods-dead-quiet-evaluation