Translation:That class is not easy.
その should definitely be "that" and not "the", though if you were really loose about translation of course it would be okay to translate any of these words as "the" some of the time. It refers to something close to the listener generally in the same way "that" does in English.
その授業は簡単ではありません。そのじゅぎょうはかんたんではありません。 I was sat with a native speaker during this question and they told me that this expression would not be used by them. They would use the word 簡単(かんたん）for easy instead. In fact both of them laughed at this expression for its poor Japanese. I put it in polite form but usually this sort of conversation would be with peers.