This thread has been interesting.
The OP asked what liabilities might exist in his circumstances. As numerous risks were identified, some more likely than others, a backlash grew about the unfairness of never being able to hire underage kids to mow lawns anymore. Except I don't recall anyone ever advocating that.
Identifying a potential risk, which is what was requested, is a different process from whether you then choose to act on it. You may learn or know of a risk and still reasonably choose to ignore it. I own a number of properties in earthquake zones, which is a known risk, but I deny adding earthquake coverage to my insurance protections. I believe the chance of an earthquake hitting is too small compared to the increase in premium. So far, I'm a hero-- no damaging earthquakes yet!
In this case, the OP felt adding umbrella insurance coverage was good, but discarded concerns of violating his employer's or school's policies. All in all, not a surprising result to address some concerns but leave others behind.