My homeroom teacher always agrees with the mean girl? what do I do?

Today, after everyone was done with the Stanford Nine Test; so, we played a game, "Heads Down, Thumbs Up", it's a game, where someone touches your thumb, and you have to guess who did it; and you can't peek. And I guessed my clmate. Then, it started this whole argument, and the mean girl, Rosanna said, "You guys are so stupid!" then the teacher agreed with her.