How to pronounce "Question"?

I'm a native english speaker with a slight east London accent. I pronounce it kwes-chun, as do most people I work with.

I have only heard question pronounced kwes-shun after the speaker has consumed vast amounts of alcohol and is slurring their words. Additionally, posher accents are more clipped and it can sound like kwes-tee-un, same as kwes-chun but with less "ch".

In all standard dialects of English, question is pronounced /ˈkwɛs tʃən/.

The closest case to pronouncing question with the sh-sound rather than the ch would be how some speakers drop the t-sound in actual so it's pronounced /ˈæk ʃu əl/. This is simply one unvoiced stop (t) assimilating to another (k).

Speakers of AAVE (African American Vernacular English) often drop final-t in words like breakfast, so if quest is pronounced ques', then que-shun shouldn't be far behind.

Be that as it may, the standard pronunciation of question still obtains, regardless of what you're teacher says, and it isn't /ˈkwɛs ʃən/ or /ˈkwɛ ʃən/.