Although not a classmate, Mr Porter became majorly implicated in Hannah's story towards the end of Season 1 when we discover that she told him she was raped by Bryce, and is having suicidal thoughts, and he did little to really help her, a combination of his hands being slightly tied and being distracted by his own problems.
Now, though, he has the tapes, and is going to hear in excruciating detail everything that happened to Hannah, while he also knows from Clay of Bryce's guilt. So what will his next move be? Will he submit the tapes for evidence? Will he even listen to them in full? Can he possibly keep his job once all of this out?