"As I mentioned in the last episode discussion thread, it's easy to guess why Haruka's reason why she escaped her own class. Rewatch her arc from the first season if you don't remember. Another hint: Her regret was solved in the dream world, but it's still a problem in the real world. So I wonder why she didn't want to stay with her own class? Do you remember what her regret was?"
This wasn't enough hints? >w>
I even told you how you could find out. Well... another way is to play the visual novel after finishing the last episode of Refrain. You might experience a different feeling as you go through the same story again with more details added. Since you now know what the secret is. :3