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Wiktionary
redshirting

vb. (present participle of redshirt English)

Wikipedia
Redshirting (academic)

Redshirting is the practice of postponing entrance into kindergarten of age-eligible children in order to allow extra time for socioemotional, intellectual, or physical growth. This occurs most frequently where children's birthdays are so close to the cut-off dates that they are very likely to be among the youngest in their kindergarten class.