We do not expect class members to help one another during tests. -�each other is a reciprocal pronoun
the phrase each other is known as a reciprocal pronoun because it shows a bidirectional action. for example, if bill and ted are being excellent to each other, that means bill is being excellent to ted, and ted is being excellent to bill. theyre practicing what you might call excellence reciprocity.
one another is a reciprocal pronoun
but bill and ted arent talking about being excellent just to bill and ted; they want each person in the world to be excellent to every other person. according to some grammarians, if were talking about more than just two people, we should use a different reciprocal pronoun: one another. in other words, bill and ted should more properly have said, be excellent to one another.
credit: quick and dirty tips.