Prefect Satine Spark finds any excuse to punish the other 6th form girls, like Masie Dee.

Poor Masie has been spanked by Satine for a uniform breach (and we see a brief extract of this).

So when the headmaster reveals that he has discovered that Satine has been sending boys pictures of her boobs, he asks Masie to decide if Satine should be gunged or not. Of course Masie says yes. So Satine goes into the tank in her school uniform and gets a double gunging of green gunge all over her pretty uniform. Once Satine is totally trashed, Masie takes pity on her and the girls make friends again by taking a naughty boob flashing selfie together. The last few minutes show the gunging from a separate camera angle.