Rules FAQ

Removing a character from a challenge (with something like Areo Hotah) does not automatically stand a character any more than standing a character (with something like Castle Black) would automatically remove it. In this case, however, Highgarden specifically references both standing and removing the character from the challenge.

The "stand" part of the effect and the "remove" part are not dependent on each other (they're not tied by a "then"), so as long as at least one part can happen, you can trigger Highgarden. This means you can target an already standing character (like a Jaime that didn't kneel to attack, for example).