I struggled with this when I was a school teacher. English lacks a good way to clarify you are addressing a group vs one person, which comes up a lot in a classroom. “Class, you…” is clunky, “You guys…” has obvious issues, and y’all or any other contraction is generally considered bad grammar. I generally went with y’all. Kids would laugh about it, but that seemed to help get their attention.
Yes, this is a case where you aren't forced to use "you" ambiguously in that context.
No, because "you in the back" could refer to just one person in the back, instead of several.
If you meant to address one person, you'd have said that one person's name, instead of voluntarily introducing ambiguity to the situation. Context & body language also makes this obvious. If you meant one person, you'd be making eye contact with one person instead of a group of people, etc. Students also know if they're paying attention or not. "The back" is not a specific area.