My teacher said the word 'grownup' can become 'grownups' but if it's written as two separate words 'grown up' with no hyphen it changes into 'growns up'.

It doesn't sound right to me and I tried to google it but couldn't find anything useful.

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    That's incorrect. It IS valid for certain fixed phrases which have the adjective following the noun. E.g. an attorney general is the attorney who is general, so the plural form is attorneys general. But with grown up (which I would never not hyphenate, by the way), there is no grown who is up, so growns up is not correct. – nollidge Jan 27 '20 at 21:14
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    "It doesn't sound right" is basically the key. Spoken language is primary, and that's just not something that you'd say. So whatever spelling convention you then choose to follow or invent, whether it dictates the usage of one blank, no blanks, or three blanks, there's just no S in the middle. Only at the end. (Conversely, if we collectively decided that "father-in-law" now has to be written all in one word, "fatherinlaw", then the plural would still have to be "fathersinlaw" with the S smack in the middle. Because "fatherinlaws" just isn't a thing that we say.) – RegDwigнt Jan 27 '20 at 22:11
  • @nollidge Good point. Interesting though that with the verb "grow up" we conjugate it "grows up", "grew up" etc - never "grow ups" or "grow upped". But with the noun never "growns up" – WS2 Jan 27 '20 at 23:25
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    @RegDwigнt: But we do say "in-laws". Language isn't always logical, and in-laws isn't a true plural form, so that's just something we have to live with. – MSalters Jan 28 '20 at 14:05

Whether you hyphenate it ("grown-ups") or write it as two words ("grown ups"), the s goes on the end. Google Ngrams reports no occurrences of "growns up".

Grammatically, if "grown up" is regarded as a two-word noun phrase, then it is a noun phrase which does not contain a noun; this makes it a headless noun phrase. If a headless noun phrase has a plural, it is formed just as if it were a (single-word) noun. So "grown ups". (There are other sorts of headless noun phrase besides this sort.)

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    I would add that this seems to be an example of overgeneralising from compound plurals like sergeants-major, courts-martial and fathers-in-law. – dbmag9 Jan 28 '20 at 8:47
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    @dbmag9 Notably, all of those examples can - at least in the root form, even if no longer used that way - be reordered: a major sergeant (as opposed to a minor one); a martial (or military) court; and someone who is, in law, your father. This goes back to parts of English - especially upper-class or military - deriving from French, where the adjective typically follows the noun. And you pluralise the noun, but English adjectives tend to be invariable – Chronocidal Jan 28 '20 at 9:08
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    Another example similar to what the OP's teacher mentioned might be "passers by" which is used very often. But I've never heard "growns up". As you said, this is likely an over-generalization from irregular examples – Oztaco - Reinstate Monica C. Jan 28 '20 at 16:12
  • @Oztaco As to "passers-by", it's only an accident of English that this verb has an agent noun at all, and that it is "passer-by" -- that is, with the particle at the end. German, for example, does it differently: it has separarable verbs but in their agent nouns the particle is a prefix (e.g. vorreiten, er reitet vor, Vorreiter). And English does likewise with some phrasal verbs: those who look on are onlookers. – Rosie F Jan 28 '20 at 17:26

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