How to say average in Japanese.
How to say average in Japanese, like average student or ordinary.

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Answer

I would say "average student" 平均的な生徒/学生[へいきんてきなせいと/がくせい]
I think we have several translations for "average/regular/ordinary" depending on the context:
平均的な、平均の、標準的な[ひょうじゅんてきな]、標準の、普通の[ふつうの]、一般的な[いっぱんてきな]、一般の、etc
When they are used with noun, we sometimes change the way if saying like Naim said: 
(with person/people) 一般人[いっぱんじん]、凡人[ぼんじん]、平民[へいみん]、普通の人[ふつうのひと]、etc

I hope this was helpful.

Answer

average = 平均　へいきん
regular person ("average Joe") = 凡人　ぼんじん