/d/ in pendant is articulated after and before the nasalized vowel /ɑ̃/, it’s entirely possible in an allegro or allegrissimo speech tempo for the soft palate to stay lowered throughout and for /d/ to become thus nasalized as well, in which case, it would be realized as [n], so that we end up having [pɑ̃nɑ̃] instead of [pɑ̃dɑ̃].

It’s probably one of those things that happen in connected speech when people speak fast, so I wouldn’t say it’s not correct but it might be considered as slurred speech by some.

I add this answer because the former ones seem to me a little confuse. The answer to your question is quite simple: Absolutely no French speaker will ever skip the “d” of “pendant”. You will never hear “pen-an” from a French mouth as your heard it from your teacher.

I will add that your example “pendant une six semaines” is not correct French. You can just say “pendant six semaines” or “pendant une période de six semaines“.