Sébastien is also not completely certain it is right to go with an adult he doesn't know, but obviously M. Joly is a good man.

No, not obviously. It's not good to make assumptions. But the evidence suggests he's a good man. He is kind, he answers questions, he is not suspiciously confident of his answers even when confidence isn't truly called for.

"Very well, monsieur," Sébastien pronounces, after a moment's deliberation. "I think it's important to find out whether the magic made us younger or made us switch places." He smiles. "Do you think grownup me is at school now?"

That's a nice thought. Maybe grownup him could tell the master to stop being so mean and bullying the shyer, slower boys.