There are probably valid reasons why pencils should be allowed but surely "the children like pencil because you can rub it out" is not one of them.
They need to be taught that in this context crossing out is fine. It is first draft work and the crossing out shows that the children have been working - checking and changing. If they have erased, you have no evidence of their editing ability. If they are compelled to use pens, there should be the same rule as in public examinations - no ink erasers allowed!
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