I have a problem with the whole "thought they left together." If you see a kid walking out the door with his mom, and the door shuts behind them, and you don't see either one of them the rest of the day, you can say "They left together." But if the mother later comes back and says "Where is my son? I left him here," you might at that point say, "Well, I thought they left together." You were sure of something when you saw it happen, but then someone challenges it later (which I believe is gaslighting in this case) and you are forced to qualify your belief with "I thought" rather than "I'm sure."
The teachers had reason to form an opinion from what they witnessed. I would not be so quick to discount it because someone (who may well be lying) challenged it. No, the teachers probably did not videotape the two of them driving away together. Why would they? But they saw what they saw, and I believe it was a significant event that gave them reason to believe Kyron left with Terri.