The paper you actually need
The diploma itself is a single printed sheet, and schools rarely reissue it. Ask the school office in Milan, or the provincial school authority, for a certified copy or the substitute certificate that carries a live signature. Anyone asking how to legalize a High School Diploma needs a signed paper, because a seal can only sit on a signature. A laminated original you cannot risk losing is best left at home.
Ask for the marks or the final grade on the same sheet, or on a second one, if the reader abroad wants them. Universities usually do. Getting both signed at the same visit saves running the chain twice. Check that your name reads exactly as it does on your passport.
Which desk stamps a school paper
A school certificate is an administrative act, so the Prefettura (Prefecture) of Milan legalizes it. The office holds sample signatures of school and authority officials and matches yours before signing. Court rulings and notarial deeds take the other road, to the Procura della Repubblica. The consular legalization of a High School Diploma in Italy comes after the Prefettura stamp and depends on it.
The mission at the end of the chain
Then the file goes to the embassy or consulate of the country where you will study or work. Their officer checks the Italian seal, stamps the sheet, and may log it. The embassy legalization of a High School Diploma in Milan is booked with that mission under their own rules. Several also want the university offer letter or the job contract with the file.
Cost and timing
The Italian stage needs a €16 tax stamp (marca da bollo). Buy it at a tobacconist before you queue, because the desk keeps none. The mission then adds a consular fee it sets on its own, and it can change that figure at any time.
Allow 10–15 business days for the run. University deadlines are fixed and unforgiving, so a lane of 3–5 business days is open when one is close. The mission's diary rules the last step, so book that slot the day your offer arrives.
What a university or employer checks
Students legalize a High School Diploma in Milan for a university place, a visa file, a licence to train, or an employer that checks every claim on a form. A university reads the subjects and the final mark. A visa desk reads the dates and the school name. An employer often just wants a sealed sheet on file. Ask each of them what they will keep.
Some universities want the marks turned into their own grading scale, and that is a separate job from the seals. Ask the admissions office whether they do the conversion themselves or expect a statement from an outside body. Sorting this out early avoids a rushed extra step in the last week.
If the school has closed
Schools merge and close, and the records move to another school or to the provincial school authority. Ask that authority who holds your file now, then order the certified copy from whoever does. Their signature is the one the Prefettura will match, so it has to be a live one on paper. Allow extra days for this, because the search takes time.
Acceptance notes
Only the destination can tell you if a mission must be involved, so write to the receiving authority and keep their answer. The embassy attestation of a High School Diploma in Milan is a wasted week when the admissions office is content with the Italian state stamp. Ask them which language the sheet must be in as well, and whether the translation is bound to it or sent separately.