There's a difference between testing for "capability" and testing for what is actually learned.
Standardized tests, if that's what you're talking about, don't measure capability at all, and often times they don't measure what's learned either.
Teacher should be able to tell capability based on observing the student's class participation and through interaction with the student.
This means lumping smart ones together without dumb ones, and dumb ones lump together without smart ones. It's ok with you? It is with me.
Why do you have to lump anybody? Why not have the "smart ones" be responsible for helping the "dumb ones" in class? Peer pressure is a powerful tool.
And the biggest problem with "lumping" students is that often the best teachers are assigned to the best students instead of the ones that need the most help.
If anybody needs to be lumped together, it's the parents that don't instill in their children the work ethic to study hard in school. They need to be lumped together and given a stiff kick in the pants.
Tracking, unfortunately, raises the "hey why are all these students of that ethnicity" political correctness antenna, and all is lost without debate.
It's not about ethnicity. It's about the family's emphasis of the importance of education. Cultural attitudes obviously have a strong bearing on this, but it need not be the case.
Please show me a viable implementation that isn't also politically incorrect, where correctness is the philosophy that all students are the same.
EDIT: and banish all private education, make it all free, please... thanks.
Nobody thinks students are all the same. Where are you getting that from?