ezi18n function was deprecated some time ago and subsequently removed from eZ Publish.
You need to either find a newer version of that extension that uses the new functions for translating the text or hack the extension and replace calls to ezi18n with calls to ezpI18n::tr function.
I'm the original author of ezclasslists extension and I must admit that I have done nothing on it for months (almost 2 years :() even if there are some bugs to fix like this one and I have several improvements in mind...
To fix this, Edi is right, you have to replace all ezi18n() function call in PHP by ezI18n::tr(). I'll try to provide a new version quickly...
Building off what Damien said : go the open-source way !
You can make the modifications (which you will need anyways), and after having become a member of the project (click the right-column button once logged-in there : http://projects.ez.no/ezclasslists/team/members ) please contribute back the modifications.
That is the way to rock in the eZ Community.
By the way : welcome aboard !
Cheers,
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Nicolas Pastorino
Director Community - eZ
Member of the Community Project Board