If you want to get a legal job in a school, you need to know the Greek language and have a certificate to prove it.
In Athens you can learn Greek at Celt or the Hellenic American Union. When you learn Greek you will sit exam for the Certificate of Attainment in Modern Greek (from the Hellenic Ministry of Education and the Centre for the Greek Language). This certificate is available at four levels.
In other places, such as the islands, ask the PALSO association www.palso.gr about schools offering Greek lessons in each area. In Corfu town, for example, there are at least 3 or 4 Kendra Xenon Glosson that teach Greek now. However, these are recent initiatives and while big cities like Athens and Thessalonica can certainly help, it may not always be possible to find language schools in smaller places. Sometimes distance learning courses can help. A pioneer in this field, in Greece, is the Hellenic American Union based in Athens.