This has nothing to do with a monopsony. In a perfectly competitive market, wage == marginal product.
A monopsonistic situation can drive wages below the marginal product. A monopolistic situation (e.g., multiple school districts, 1 union) can drive it above the marginal product. What I described is simply the ideal situation.
Yes - but the reality in most large cities (where the problems are) is 1 union and 1 employer. Once you get to the suburbs, there is a lot more mobility amongst the teachers. If they don't like a school system, another is 5 miles away.
But let's say there are multiple employers competing for the superheroes, each with their own bad guys to hunt down... Then you can get paid more.