The signs being held up by the student protest are not particularly clever or unique for the time, but the message is strong. One is easy to explain.
The one on the left reads DALJVAL KAKULTAN (Dzhahlj-VAHL KAH-kool-TAHN) or, basically, “Tests Flunked”
The other is an old pun turned into a math formula. CU + RU = RID (choo mm-NZEHTH roo SNAH-jrr reedz) or “Y’all (feminine) + Us (any) = Enemy” where “Enemy” is actually a literal form of “Cousin” (genderless) due to the insult of any kinship besides siblings being impolite in some old-fashioned “proper” school teachings. The boycotting students use the feminine “you(s)” ironically, since that indicative word is often used by patriarchal teachers to refer to any non-males in a given class. It hits the point in Sonia’s Pærthian culture due to the arrogant way the educators have enforced certain poisonous divisions between ages, genders and so on. There is also a certain poetic wit and funny rhythm to reading it as a formula.
I guess some students find that throwing fruit is funnier and more on message.