Dismissive term for college diploma

You'll sometimes hear "a piece of paper" used this way; see https://www.google.com/search?q=went+to+school+for+a+piece+of+paper for plenty of examples.

(But as choster notes: "The expression can be used to dismiss any kind of official document, however, not just diplomas or other educational credentials: marriage licenses, operating certificates, travel visas, building permits, arrest warrants, property deeds, constitutions.")

It's often said in English to be a 'sheepskin'. https://www.merriam-webster.com/dictionary/sheepskin

Because at one point, diplomas were printed on parchment made from sheepskins, the term “sheepskin” used to refer to them (first recorded in 1804): see the bottom of the page http://www.dictionary.com/browse/sheepskin

As an American, sheepskin came immediately to mind, but I think the OP is referring to a worthless Diploma (i.e. Not worth the paper it's printed on).

A prime example in America would be a Diploma from a DIPLOMA MILL "An institution of higher education operating without supervision of a state or professional agency and granting diplomas which are either fraudulent or because of the lack of proper standards worthless. - Webster's Third New International Dictionary". However, I could not find a good slang term for the Diploma itself, just the institution it comes from.