Well, sure they're assholes. But they're pragmatic and were able to think about the dangers of what they were about to go through. The asshole school kids were more mature than a dude 10 years older than them and already working in society.
There is no excuse for this dude's stupidity. He wasn't aware of the dangers he was in and thought it was just another walk in the park. And this while thinking that his skills are useless. What was he trying to do? Relying on the kids to protect him? He's lucky that he even had any supplies at all.
If Ai-chan is there, while Ai-chan wouldn't have stolen his supplies, Ai-chan would've left him behind too. It would be different if he is aware of the dangers and try his best to be of help. But all he did so far was slow everyone down. He deserve being left behind.
The kid was right, at least the kid didn't kill him or use him as bait.