Hi Xrysa,
I'm not sure what's the...ahem...current reality in Greece, but one would expect that in order to teach in public schools, a university degree is a must. The Proficiency Certificate from Michigan that you mentioned doesn't count as one; you probably need a Master of Arts degree with English as a major subject (perhaps even as a secondary subject, I'm not sure). Quite probably you also need some Pedagogic classes, either as a part of the degree or seperately.
Abroad, in most of Europe at least, the situation is so that these kinds of jobs pass through the same channels as the rest of the jobs (i.e. interview, etc.). But in Greece, with the moronic waiting list system (in Greek: epetirida), it doesn't matter whether you graduated with honours and at the top 5% of the class or you hit rock bottom and barely graduated at all, because, as you well know, you just have to "wait your turn"..
Sorry if I sound pessimistic, but I've had it up to here

with the Greek educational system
