気が 散る means to become distracted. Your 気 (spirit) does the action of 散る (to scatter).
Ami is used to getting the top grade on mock exams, but when another name starts tying her scores, she begins to get flustered. Her friend Minako suggests they investigate the person behind the name, but Rei isn’t so sure.
亜美ちゃん 気が 散って 成績が 落ちちゃうわ！」
- “The way things are, Ami-chan’ll be distracted, and her grades will drop!”
始めたってのがバレた 時の 方が 亜美ちゃんの 気が 散ると 思うけど」
- “I think Ami-chan will be more distracted when she finds out we’ve started something like this.”