This situation, actually, should really be sorted out by the instructor, and his decisions would show the quality of his work. Lesson horses should not just be switched by the liking of the students, but be assigned according to the specific needs of each horse and student. A good instructor should also prevent such personal misunderstandings that sometimes come up between students regarding who rides which horse - it should all be handled by a knowing supervisor.
Not caring about who rides which horse... "Dead broke" horses, who give their riders such hard times that they don't even want to ride them (regarding Jelly)... Inadequte assistance and tutoring - regarding always having the same problems with one horse (Kelly) and not showing any progress - and letting to get away with that by simply changing horses!.. All of that doesn't look good.
Girl, I've riden tons of hard lesson horses in my time - the stubborn ones, the flighty ones, the uncooperative ones, young ones, old ones, all kinds of them. Sometimes I felt like giving up, like when I was given a particularly hard mare - and when I asked for a different horse I was told that I will keep riding this mare until I sort out at least one of my problems. And I did. All of that gave me extremely valuable experience in dealing with different horses - riding only an easy and likeable horse would have brought me nowhere! So appreciate what you're getting and stop acting like a spoiled child. At times like these...we just have to grow up.
And I understand the fun side of getting to be at the same lesson with your best friend, but you will get mutch further riding-quality-wise, if you just concentrate on yourself and the horse. Otherwise, you'll always be too busy to learn how to cooperate with Kelly.
I have come a long way, to surrender my shadow to the shadow of my horse.