Sunshine Ep 4 Thoughts Adding to this, got a little theory. The small room that Mari gives them and the lyrics on the whiteboard. I feel like Mari and the third years probably tried making an idol club, but something happened. I have two ideas on this, one is that Mari leaving Japan messed everything up for them. My second is that they weren't able to come up with enough members, and they were forced to shut down (and maybe Mari leaving because of that). I figured that's why Mari wanted to approve them even though they only had 3 members; to give them the chance that she didn't, but also tested them at first by telling them to fill the gym. It's also probably why Dia was so adamant about not letting them make a club. The fact that she also talked about not having a composer in the school might show that it might've been a problem they faced with their idol club.
Idk, this is all just me imagining / theorizing but I found it interesting :P