Honestly, the whole idea of doing this slumber party during the Beach trip was pretty stupid...I mean they could have just done it on a weekend at Ayane's house or something. Ayane could literally offer Yumi a buffet there, and no one would have found out. It's like Ayane wanted them to get caught. Perhaps she was wanting to see the Shapeshifter/Himawari?
Nah, I think it's just practically more feasible to get everyone on their list to show up when there already is a class event to
force their attendance. It might be unimageably difficult to plan an overnight outing with someone you don't normally hang out with under a reason you can't quite explain, and then have people actually show up for you.
(can't even ask two friends to show up on my birthday...)
Besides, there really isn't a tangible consequence if somebody else finds out. There will either be more girls to test their methods or just more girls caught on the loop. Even Chika's case hardly qualifies as what she saw wasn't a part of the intended sleepover program; I don't think things would go down that poorly for her if Chika spotted Sensei earlier when he was still in the room with everyone.
More importantly, since Ayane is quite sure about sleepovers like these would trigger a reset immediately after, she likely prioritizes figuring out the task at hand as there likely won't be any unwanted continuation spawned from her plan.