What grade level are you talking about? Is your role the parent of the marginalized student, or perhaps an aide in the teacher's classroom? If you're the parent, have you been able to go into the classroom on a regular basis to verify what's going on? It's vitally important to verify the situation, not go on the word of just one person.

If, in fact, you confirm there's a problem, you can document what you've witnessed in a letter to the principal. Teachers should keep favoritism and biases out of their classrooms....

In any case, I can't help but wonder what's happened to cause the one child to be marginalized? If there are behavioral issues (let's face it, it's easier for a teacher to favor a child who always does what's asked promptly, without argument or disruption) then my question is what's being done to provide positive reinforcements to the marginalized child? If there are academic weaknesses that one student has, versus another, then they should be addressed. No child's needs should be ignored.