The reason that the animal attack on Petra is so dangerous is because of her response. Because Petra is a telepath, her cry for help was not an auditory scream. During the attack, she let out a loud telepathic cry for aid. What that cry did was go to every other telepath in the area. That in turn brought the entire telepathic group together. It happened to be the group's first physical meeting, which is cool for them, but dangerous at the same time.
The reason their impromptu meeting is dangerous is because it aroused suspicions as to how that group of young people came to be together at that place and at that time. Remember nobody heard an actual cry for help, so a group of kids that just happened to come together at exactly the right moment to rescue someone is odd. Petra and David's society doesn't tolerate odd. Any genetic oddity is cleansed from society. In other words, they are killed or banished. Telepathic ability would definitely classify David, Petra, and the rest as Deviants. That would put all of their lives in danger.
The plans that they make are not complex. The group believes that if anybody had any really large suspicions or proof, that a formal inquiry would have been started already.
". . . 'they can't know much, or they'd have called an inquiry already, and they'll have to be pretty damn sure of themselves before they do call one. The inspector isn't going to put himself in a weak spot with your father if he can help it . . . "
The group agrees to go back to their lives and act as normal as possible, but to keep a very watchful eye out as well. If anything suspicious comes up, they are all agreed to run for it. If that means killing someone, then do it, because if they are discovered Deviants, then they will be killed. Lastly, David is told to kill Petra if he can't save her from getting captured.
"If the worst comes to the worst, and you can't save Petra, it would be kinder to kill her than let her go to sterilization and banishment to the Fringes — a lot more merciful for a child. You understand? Do the rest of you agree?' Their agreements came in."