i know they're following the novels (and afaik, Konno-san is still writing 'em -- 29 or 30 vols thus far published in Japan.) i'm pretty sure season 3 encompasses the bit where yumi and her class go to italy, so we've got some plot yet to go through.
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according to wiki, season 3 has 5 ova eps:
http://en.wikipedia.org/wiki/Maria-sama_ga_Miteru