Which of theese phrases are correst?
1. He is in the back row.
2. He is at the back row.
You can also say 'He is on the back row'.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
If I'm pointing myself out in my school photo, I'd say 'That's me - third from the left on the back row'.