Portuguese translation: Diploma de Fim de Estudos Secondários
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Explanation: I don't know Portuguese, so I will answer in English.
The "diploma of maturità classica" is a school leaving certificate achieved after 5 years of studies in the "Liceo Classico", i.e. a high school in which the curriculum is characterized by Ancient Greek and Latin studies.
I hope you can now find the equivalent in Portoguese.