Mean Girls drops three fantasy sequences on us, each one straight from Cady's head. One involves a trio of potential "junior Plastics" getting leveled by a school bus. In the other two, Cady pictures herself and her classmates going positively primal.
These sequences, where Cady imagines North Shore students as jungle animals, symbolize Cady's struggle to understand their culture. Their rituals, like hanging out around a mall fountain and or being passive aggressive over cheese fries, seem completely foreign to her, so she relates them to what she's knows best: the animal kingdom.
Cady's fantasies also symbolize her rejection of North Shore's tribes and traditions. Her classmates don't come across well in either fantasy. At the mall, they turn into monkeys, picking each other's nits, dragging their knuckles, and toppling one another into the watering hole, a.k.a. the fountain. Cady sees her classmates as less evolved than she is.
In the cafeteria, Cady and her classmates are straight-up violent. Cady imagines herself pouncing on Regina like a jungle cat, trying to tear her apart, while Aaron bares his teeth, practically foaming at the mouth, and Gretchen claws at the table. Here, Cady's rejecting the underhanded way that Regina dangles Aaron in front of her like red meat.
In real life, Cady has to be the bigger person and take it. In Cady's imagined animal kingdom, she'd settle things by going straight for the jugular.