This. I took Calc 1 twice. First time was during the day in a big auditorium style classroom. The prof had a pompous attitude, like he didn't even want to be there. I dropped the class mid-term. I re-took it in the evening. Smaller class size, mostly "non-traditional" students. Prof was awesome. Frequently stopped the lesson to re-explain things and made sure everyone was understanding, stayed after class to help students. Because of that class I actually "got" calculus.

Yes, and I believe many colleges make it intentionally difficult. My Calc I class was given by an instructor with a heavy Asian accent. I had to struggle to understand every other word.

My Chemistry for Engineers class was in a large classroom that was under construction.

Needless to say, I went for MIS instead of CS/EE. I ended up taking many CS classes, so it worked out well.

On a side note, teaching is really difficult. Knowledge of the subject is the minor skill. My parents were teachers and I considered it in college but didn't have the patience.

Weed-out is a really bad way to look at teaching. It puts people in the mind set looking for excuses to get people to fail. The attitude should be teach-in.

If someone passes your entrance tests then in most cases if they fail either you failed to develop the ability that you decided they had, or your entrance test was wrong.

The UK military realised this a while ago and describing any activity as being 'weed-out' is a big no-no.