Can't enter horse in Riding School?

This might be a kind of dumb question, but I'm not sure what I'm doing wrong... I rescued a horse today (this one: https://www.equiverse.com/horse.php?id=704846) And I want to enter him in a riding school, but the dropdown list in the Riding Schools is showing, "No Horses Available". He's in a division and he's been boarded, so I'm not sure what I'm doing wrong? Is there a way to find out if a horse has already been entered in a show or Riding School that particular day?  

He’s likely already been shown or completed his riding school hours for today. You can tell if the blue ribbbon is filled in from your division page.

Ah, that was it! I somehow never thought of looking at that blue ribbon haha! Thank you!