There are some crappy teachers out there, no doubt.
And Uni/College lecturers too.
As long as you do the work to the best of your ability, and you can stand by and back up your convictions, then I believe you have every right to challenge on your own behalf.
More than our fair share of crap lecturers at UWE. All foreign too, funnily enough. No doubt drafted in on a cheap wage. A couple of Asian ones who can barely speak English shouting words at you and getting flustered when you ask them a question. Way too many non-English speaking lecturers here (on our course at least, can't speak for the others).
We had an Italian lecturer last year who talked in an incredibly soft monotone voice, and the entire tutorial was her reading the homework questions and answers word for word off the board. Ask her a question and she would just repeat the answer. End of the tutorial she would literally just stop speaking and not move. Then just stare at us whilst we looked at each wondering if we were allowed to leave or not. Thankfully we don't have that module anymore, although she does take a lecture after one of ours. I feel for those students who have to put up with a lecture. I'd be surprised if anyone actually turns up.
She sounded a lot like Consuela (off Family Guy) actually. Whenever we gave a wrong answer she would respond with..