Can you put past participle adjectives in plural form?

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.

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.)