it's an insane risk. I work for transportation in k-12 and I would never transport kids without several feet of paperwork, it's difficult even with that. you have to have a lot of policies about what to do if and when anything happens, they won't get out, they have a medical emergency, they refuse to use a seat belt, they scream racial slurs, they start a journey of self discovery, they hit their head on hard objects for fun, they punch windows aggressively while screaming, they report criminal acts of themselves or others, they light up a Jay, they start discovering others, they forcefully insist on discovering others without consent, and so on. unaccompanied minors are a nightmare just in terms of liability and crisis management.

I wonder if the parent could've just accompanied them and returned in the same Uber, it's not even a new ride back

I don't know the details, but Lyft has teen, pink, and silver.