Third Grade Grammar

(A) Earth is one of the nine planets that ORBITS around the Sun.


OR


(B) Earth is one of the nine planets that ORBIT around the Sun.


My son's teacher: B
My son: A
Me: A

The American Heritage Book of English Usage: waffly, but either appears to be accepted, with "...92 percent of the Usage Panel preferred the singular verb in such sentences."

I shall not raise this point with his teacher. Or should I?