never gone八门:发现一个很有趣的翻译，高手进来挑战！！

Two of my deskmates, one in senior high school and one in junior middle school, are deskmates when they are in primary school.

My seatmate in high school was my junior school seatmate's primary school seatmate.

My deskmate at senior school is the deskmate of primary school of my deskmate at junior school.
要用of.用at会产生歧义。

My high school next-seat-classmate was the primary school next-seat-classmate of my next-seat-classmate at junior high.
or,
My high school next-seat-classmate was the primary school next-seat-classmate of my junior high next-seat-classmate.

*In the United Sates, you call 高中 as high school and 初中 as junior high.

**"同桌" in general should be translated as "next-seat-classmate" because in the United Sates, every student has his/her own individual chair that has a surface board for reading and writing. Therefore, they don't have seatmate or deskmate.

My high school next-seat-classmate, who was my junior school's deskmate when he was studing at primary school.

在可以的情况下，叙述同一个事物，或情况，用近义词会好些。其实next-seat-classmate，deskmate, or seatmate,应该都ok.

My high-school deskmate is the primary-school seatmate of my secondary-school deskmate.

or

My high-school deskmate is my secondary-school deskmate's primary-school seatmate.