But before judging him, I'd like to confirm that the teenagers didn't make up a fake account on the app and said it was the teacher.

Does the possibility of getting caught and punished for this register with them at all?
A few years ago I attended an anti-trafficking conference. One of the speakers was a state LEO officer in charge of catching pedophiles via social media. He was an overweight, middle-aged man, and he got a good laugh when he said something like “Hi, I’m Lisa. I’m 12 years old and I’m SMOKIN’ HOT!!!” It’s a shame such efforts are needed to smoke out such predators; I’m glad this teen was able to take one down. He might have a knack for it, and thus have a career path already opening for him.
What if I told you I think of you often? Would it make you feel uncomfortable?
What if I told you your hair style is one of my favorites? Would it make you feel uncomfortable?
What if I simply said, “You look great?” Would it make you feel uncomfortable?
What if I asked how things are going for you these days? Would it make you feel uncomfortable?
Now, when it come to explicit imagery of genetalia and discussion of such things between strangers, *that* ought to make any normal person uncomfortable at best.
This 14 year old boy handled the situation correctly. If he and the girl came from a different culture that same 14 year old boy would have gone to the teachers home and bashed his brains in. That one vigilante style act, would have ruined the boy’s life, all in pursuit of trying to protect his friend.
Most people would be astonished at the morals of students if they ever spent some time around teachers colleges.
Why wasn’t he the locsl police department involved?
In the UK, the boy would be arrested.