Continuity mistake: In the beginning, Gretchen says she's short on money, so she can't get a chocolate milk, just a white milk. But, when T.J. walks by with his food, Gretchen has a chocolate milk, such is evident from the brown carton.
Suggested correction: Gretchen tells the lunch lady that because she is one penny short, she will swap the chocolate milk (in her hand) for a regular. She puts the milk on her tray and walks off screen. Then, when T.J. walks past her, the camera pans over to Gretchen watching T.J. with that same chocolate milk on her tray she is holding. Gretchen got distracted by T.J.'s encounter with the lunch lady and stopped to watch him, but she was still going to swap the chocolate milk for a regular later on.