One day, you were putting things in your locker when you heard Lila lying about Marinette talking bad stuff about her online.

"N-no I swear guys, I didn't do any of that stuff! Even if I dislike her intensely, I would never!"

Marinette defends herself but everyone was manipulated by Lila to think that Marinette did trash talk Lila online. Adrien wasn't there, so at this point, Marinette is hopeless.