Don't worry, we'll unpack all of the DeMarcus stuff. But first we have to break down Jenna's involvement in all of this, because yes, she's the Turd Burglar . . . well, sort of.
Jenna is a wealthy student whose peers mock her after an Instagram faux pas involving Kendall Jenner, an incident that is so shameful it even causes her girlfriend to break up with her. So yes, she has motive. On top of that, Peter and Sam are able to uncover that just prior to the pep rally — aka the event where tons of students and faculty were blasted with cat poop via t-shirt cannons — Jenna got into a car accident with Lou. Lou shows them a photo of the damage to Jenna's car, which is their smoking gun. The t-shirts that were supposed to be shot out of the cannons instead of the poop can be seen in her passenger seat; that means Jenna's alibi that she was at her internship during the pep rally doesn't hold up and she was the one to stuff the t-shirt cannons with all those gross cat droppings.
When Sam and Peter finally put it all together, Jenna spills — yes, she's responsible for the pep rally incident, but only the pep rally incident. She was blackmailed into doing it by the true Turd Burglar, who catfished her using the stolen persona of a girl named "Brooke Wheeler." Jenna's online-only relationship with her got so serious that she ended up sending risqué photos, which "Brooke" then used to blackmail her into pulling off the pep rally prank. And Jenna wasn't the Turd Burglar's only catfishing victim.