In making your decision, ask yourself if each twin is capable of initiating and maintaining satisfying relationships with other children? If your answer is "yes," separation would not be warranted. If your answer is "no," then separation, perhaps for part of the day, might be attempted on an experimental basis.
You might also consider separating the twins if they are constantly being compared, and the comparisons provoke negative feelings in either twin. If one of the twins typically comes out on the poorer end of these comparisons, a pattern of discouragement may develop.
In deciding whether or not to keep your twins in the same classroom, you need to think about how much they depend on each other. If one twin helps the other excessively, you might consider separating them so that the one being helped does not become too dependent on the other.
In the case of school-age twins, it is a good idea to check their own preferences about separation. Of course, their feelings on this issue should be put into the larger perspective of the long-term development of each twin. Finally, if the twins enjoy being together, there is usually no advantage to separating them.