While I can't speak for Eyeris as to whether Dan's spell was too successful or not, I do find it kind of hard to believe that the headmistress wouldn't take Dan and Wade into her office to figure out the whole story, even if she gave them detention before she heard either side.  It just seems really irresponsible for an authority figure at the school, no less the headmistress, to hope they would stop because she said so and be on her way.