If it weren't for the "looming" reference that gives a "darker" feeling to the description (native speakers correct me if I'm wrong), my speculation would be Sophia eavesdropping or "spying" on Dylan and Emma (if you are in that path) who could be alone in the corridor setting up a possible date in the evening and maybe kissing again.
More generally, considering that the corridor seems to be empty, it could be placed during classes while Sophia has ended hers and she is walking around. And she found something happening in the corridor. Whether they are students or someone else it depends on the "looming": what could be possibly threatening Sophia? The facial expression is hard to decode..

She could have done something for the task in the corridor and now she is secretly observing the results while e.g. the students or someone find it out...