From the Airport
Via Taxi: If you are heavily laden, it is best to take a taxi. There are a number of ATMs at the arrivals terminal of the airport. The taxi stand is on the lower (arrivals) level of the airport at exit 4. Tell the driver to take you to "Kolonaki", "pano apo ton Evangelismo" (to “Kolonaki [the area of Athens where the School is located] up from Evangelismos” which is the hospital just below the School). All taxi drivers know Kolonaki and the hospital. The School is located one block up the hill from the hospital at "Souidias peninda tessera" ("Souidias 54"), at the corner of Gennadiou and Souidias Streets. The fare is a flat rate of 38 euros, unless you are traveling between midnight and 6 am, when it is 55 euros.
Via Public transportation: If you are traveling light, public transport from the airport is efficient and inexpensive, either by the metro or a public bus.
Metro: The simplest and most direct public transport is the Metro, which runs direct from the airport. Get off at the “Evangelismos” station. Tickets from the airport are 10 euros; do not forget to validate your ticket. As you leave the Evangelismos Metro station, take the exit on the left (towards the hospital) and head straight ahead to the next street, which is Gennadiou Street. Turn left (up the hill) and walk to Souidias Street at the top (3 long blocks). The School's main entrance is on your left, at the southwest corner of Souidias and Gennadiou streets.
Bus: The X95 bus (labeled “Syntagma Express”) departs from the arrivals level between gates 4 and 5. Buy a ticket from the stand next to the buses for 6 euros and validate it in one of the orange machines on the bus. You should get off the bus at the Evangelismos Metro Station after passing the Athens Hilton, roughly 45 to 50 minutes from the airport depending on traffic. If you are unfamiliar with the area, ask the bus driver or other passengers to notify you when you reach the Evangelismos stop. (Some buses are equpipped with screens that track your progress into the city. The Evangelismos stop is after Megara Mousikis.) Once you get off the bus, turn right and walk to the next street, Gennadiou Street. Turn left (up the hill) and follow the directions above which runs from the airport to Evangelismos Station.
N.B. The penalty for not validating your ticket on the metro or bus is 60 times the price of the ticket. Make sure to validate your ticket!
In late 2017, a new electronic ticketing system was installed in all public transport in Athens, called the Athena Ticket. Now, electronic (paper and plastic) smartcards are used, akin to the Oystercard system of the city of London. More information on the new ticketing system can be found here in English and in Greek.