In order for a foreign public document to be accepted by Greek public authorities, it must first be validated properly. Validation is prior to a Greek translation by an official translation service (see below).  More specifically
 
A) If the foreign public document has been issued by an authority of a country which is a party to the Hague Convention of 1961, it must bear the apostille stamp. The apostille stamp must be set on the original genuine documents (original degree, undergraduate diploma, etc) and not upon a copy.
For the list of countries that are parties to the Hague Convention of 1961, see https://www.hcch.net/en/instruments/conventions/status-table/?cid=41
For the list of institutions in each country, including our country, responsible for the affixing of the Apostille stamp, see https://www.hcch.net/en/instruments/conventions/authorities1/?cid=41
Once stamped, the documents must be officially translated into Greek by an official translation service (see below).
 
After the set of the apostille, genuine stamped documents (originals, not copies) together with the genuine Greek translation of them, must be kept available in any case the Secretariat of the School might inquire them.
 
B) If the foreign public document has been issued by the authorities of a country that is not a party to the Hague Convention or, of a country that although they have acceded to the Convention, Greece has expressed objections (i.e. Kyrgyzstan, Mongolia, Uzbekistan, Peru, Tunisia, Philippines, Kosovo, Pakistan), please contact the service of the Ministry of Foreign Affairs called “CITIZENS AND GREEKS ABROAD SERVICE AND INFORMATION CENTER”. The page of this link contains useful information that you can use before you contact the service office by email or phone.
The contact details of the Department are (service hours 9:00 am – 13:00):
1) Phone numbers: 210 – 3682040 to 43
2) e-mail: cio@mfa.gr
3) Physical Address: 3 Akadimias str., Postal Code:105 54, Athens GREECE
 
After you complete the procedure that will be instructed to you by the above Service office, genuine stamped documents (originals, not copies) together with the genuine Greek translation of them must be kept available in any case the Secretariat of the School might inquire them.
 
SERVICES AND BODIES CARRYING OUT OFFICIAL TRANSLATIONS INTO GREEK LANGUAGE

1. Paid Consular Authorities of Greece (Consular authorities are divided into paid consular authorities, which are headed by officials of the Diplomatic Branch, and unpaid consular authorities, which are headed by honorary officials )
2. Τranslators cerified by the Greek Ministry of Foreign Affairs, search here: https://metafraseis.services.gov.gr/
3. Greek lawyers that certify their proficiency of translation from the original language to Greek language.

Νo other source of translation is acceptable.