The classmates didn't react that oddly, first of all no one knew him, but also the goddess made clear that she doesn't play around and even knocked out the S tier hero. Would any other of lower tier oppose her they'd probably get disposed.
Group pressure, risk of death by goddess and no attachments to the MC all serve to create their behaviour.
What does bother me is this tired trope of an MC with self professed trash tier skills, but as a reader simply seeing their names you realize how strong they are without even seeing them in action. If you're going with this setting, at least give the MC some skills that look horrible but turn out to be great due to thr creativity of the author/MC.