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.

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