High schooler isekai sounds silly, but with some thinking it's reasonable for the summoners.
You don't want someone smart or with authority, like a scientist or military figure, because they would be aggressive and knowledgeable enough to establish their own clique and try and create an organization that could potentially be at odds with existing ones, and also have the ability to leverage the world's magic, population, etc. against you.
You want some pretty face with a hero title to slap onto, without enough life experience to question the whos, whats, wheres, and whys. That way you can keep them under your thumb.
Of course, in most isekais both the protagonists and the villains like to pass around the idiot ball, so it's not really important anyways.