@FelipeOliveira Most dictionaries will list it as 込む because that's the traditional answer, but I think it's usually 混む in popular usage today, and many dictionaries now list 混む as an alternative. A few more progressive dictionaries like 明鏡 and 集英社 put that meaning under 混む instead, reflecting modern usage. Also, こむ wasn't given as a reading for 混 on the original 常用漢字表; it was only added in the 2010 revision.

@FelipeOliveira Well, I don't want to tell you what to do, but I think I'd probably write it the way your teacher suggests.

Incidentally:

(nlb.ninjal.ac.jp/headword/N.08647)

By the way, that data might look different if it were up-to-date with writing from 2020. Most of the subcorpora in BCCWJ go to 2005, with a few reaching as late as 2008, but all of them predate the change to the 常用漢字表 in 2010.

People already tended to use 混 before then, but some editors or publishers may have avoided it.