The Junior Plastics attended North Shore High School. Damian Leigh, in his Senior year at the end of the movie, saw these three pretty and stylish girls walking together snobbishly. Their physical appearances look similar to Regina, Gretchen, and Karen.
He dubbed them Junior Plastics, not in reference to the grade as they were actually Freshman.
Before the movie ends, they too nearly get hit by a speeding bus in front of North Shore, but the movie does show them getting hit as part of Cady's imagination.
These girls were only seen once at the end of the movie, and not for very long after that.