I'm usually really really lazy and don't get around to supporting manga until later on. But I just bought all the volumes digitally and ordered a set of physical ones as well.
Please support if you can, I love this series very dearly.
I wonder what Sensei is "wrong" about. Why Aki dislikes the rain, maybe? Doesn't seem like there's much else it could be.
Presumably, she thinks sensei is wrong that the rain will clear and there will be a rainbow. Aki is probably saying that she thinks she is going to be sad/depressed forever(maybe about sensei) and there will be no happy ending.