Fetching details from the dictionary...
sixth year of elementary school; sixth year elementary school student
私の息子は今年、小六です。
My son is in the sixth grade this year.
ピアノは小六から習い始めました。
I started learning piano in the sixth grade.
来年度から小六で英語の授業を導入します。
From next academic year, English classes will be introduced for sixth graders.