That's a fair point, though as of the episode Ladybug it seems like Alya believes in both of them, for the most part: Marinette could never have done what Lila is accusing her of, not even in jealousy, but also that Lila isn't intentionally lying to everyone and that she's either misunderstanding what happened (maybe Marinette knocked her down the stairs on accident) or in some kind of frame-job that's going on (I wouldn't be surprised if she suspects that Chloé is behind the expulsion thefts, she doesn't like anyone but particularly dislikes both Marinette and Lila, and gets away with everything).