Yuuka seems to be surprised. Maybe she got a love letter in her locker? Maybe she realized that she graduated many years previous and is wondering why she is still having a dream about being in classes at school?

I dunno, the way the eyes are extremely contracted screams incoming yandere time. Or she just realized something has gone horribly wrong.

It more like "I have to put my love letter in his shoelocker but I don't have enough courage yet to do it." kinda face to me.

According to the caption, she is stunned, because for the first time since she was born, she got a love letter!

Poor Yuuka, but now, everything will be good... TT_TT

(And then it turns out it was just a prank => Mass Murder)

there must be a shrunken head in that locker and the shock is overwhelming

She realized there is no flowers in the lockers.

Someone put flowers and a love letter in her locker. She doesn't know how to respond because she would either have to accept his love or kill him for picking the flowers off the field/garden.

lastsinz said:
Someone put flowers and a love letter in her locker. She doesn't know how to respond because she would either have to accept his love or kill him for picking the flowers off the field/garden.

This

I think there is an Acadia or Anemone in the locker

It's a dick in a box.