Boys see through her dress.

As she stood in front of the door, the teenage boys seated surrounding her began to stare. Marinette could hear them whisper, and she froze as she faintly heard one of them say "..hot girl in the white dress.. pink underwear", which made her blood go cold. She remembered that she wore a matching pink set of underwear, and there was no other girl wearing a white dress on the bus.

'How did they know!?' she wondered, in a panic. Trying to be discreet, Marinette glided her shaky hands over the back of her dress to make sure it wasn't stuck or lifted.

And then she remembered. She forgot to put on a shift this morning. They could probably see her underwear through the white fabric of her dress.