A student who comes into the 6th grade at a kindergarten reading level is still expected to meet grade level.
And short of an ESL or child with a learning disability, why is a child with that reading level entering 6th grade?

Screw social promotion, embarrass the hell out of them and they might have a newfound impetus to learn. It'd look damned silly for an 11 y/o to still be in kindergarten.