Perfect grammar is less important than comprehensible semantics for all but the most niche linguistic applications.
One of these "niche linguistic applications" is language learning.
English is uniquely riddled with exceptions to rules linguistically. Teaching inflexibly only makes students flex around you.
That is "correct" grammar, English as she is spoke.
That's also grammar that's fit to get you harangued and chided by other 4-9 year olds on the schoolyard. Social outcomes are the primary means by which language is meaningful.