Some words give rise to endless debate. Platypus is one of them. Those who argue for platypi are mistaken in that platypus is Greek, not Latin in its origin. The common sense plural is platypus. We have no problem with fish, deer, duiker, sheep and countless other singular plurals, so let's add another to them.
Phrases tend to cause some problem.
Strategic Directions for the University of Tasmania in the 1990s which was published in 1993. The preferred plural of 'head of school' is 'heads of school' The abbreviation HoS is singular or plural depending on context. The University employs inclusive language but seeks to maintain readability and common sense. The following are some general rules. The practice of refering to a generic noun in the singular by the pronoun 'they' should be eschewed as both ungrammatical and open to wrong interpretation.