Sudbury Valley Schools are the gold standard as far as I can tell from my research. (I spent a number of years as an education/students rights activist; built and ran the country's most famous education blog back in 2009; and have otherwise been doing advocacy work for a couple decades) They are fully democratic and focus on producing empowered self directed citizens who understand how to share limited resources and…
What if students (or whoever in a power sharing dynamic) are making decisions that are bad for them? How do you veto a bad decision?
In those kind of pedagogies (mostly Freinet for me), the adults ultimately do have a veto power. However, when you give kids responsibility, something funny happens - you find that they tend to become very reasonable, and you very rarely have to use veto power. More often than not, it's more renegotiation ("We can't do that today because of X, but what about tomorrow?") than true veto. And provided that you explain things clearly and don't disregard their intelligence, the kids typically agree readily.
Teenagers can be a little more argumentative, but the group is actually very good at taking care of the teenager or two who like to say dumb things just to create a reaction (one of the many reasons why a lot of Freinet implementations like to start and end each day with a group "forum").