You mean besides all the backup, or that we spilled there's a society by accident?Eh, I bet the Saiyans can contain this breech. There's at least half a dozen ways I can think of, some of them obviously not going to happen, but some are at least plausible. There's Seers and Sorcerers, not to mention people capable of Mind Delve (and possibly worse mental abilities).
And no, so far as Maya knows there's exactly 4 ki users on the planet: her, us, Dad, and the guy who was fighting the scout. At this point, she also knows there's something up with Jaron. But while she's doubtless guessed there's at least a few more, that hasn't been confirmed for her.
Also, the school's in a public area, and the aliens are flying in full view of the surrounding area. We're literally down the street from the (former?) Mayors office and police station, and I bet you there's someone talking about it on the internet, if not streaming it.