Honestly, Calla didn't have to be offended for her to talk that much. She talked that much no matter how she felt. But a seventh year Gryffindor probably didn't know that about her. And in this particular case, she wasn't so much offended - at least not in the sense that she was hurt; rather, she just wanted to be taken seriously.  

"Teach me. Take it slow. We can do hovering rather than on pole level if it would make you feel better," she offered to the gryffindor. She really didn't want to toss a ball back and forth so maybe her persistence would wear off. It did at home with her sisters so why not this random person at school? "I mean really if I break my arm, I take some icky potion. It wouldn't be lasting damage. Pretty please!" The arm comment was intended to be a joke, hopefully he got that.