I have a mare who's reached level 10/schoolmaster and when I go to select cross county to enter it won't show it. Is this a bug that needs to be reported or something else? I think she's like eleven too in age.

This is the one you're talking about, right?

When horses hit level 10, they can no longer show in that discipline. If you want to keep showing her, you'll have to change her discipline to jumping or dressage (I'd reccomend jumping, since she's much higher leveled in it).