I'm a newcomer just like you and I might make mistakes, so please wait for outside opinions before deciding whether you can trust my answer.
Here 成績評価の甘い is a characteristic of 授業. 甘い is used in the sense of "generous". So all together it means:
Teaching/lessons with generous grading.
Here 安売り is used in a figurative sense, meaning that something becomes easy to get (not in terms of money!). So we have:
"Cheapening" of grades. (it's getting a lot easier to get good grades)
About understanding long sentences. When something seems too complex, usually you need to break it down into smaller parts and try to see how they relate to each other. Look for clues, such as typical sentence connectors.
In your example 他方 obviously connects the previous sentence to this one. など…もある means "… like that also exist". Like what? Like what is described above, of course. So for now we have:
However, opinions like [...] also exist.
So now all that's left is to figure out the [...] part.
Seems a bit intimidating at first but again, let's see how these three phrases separated by commas (let's call these phrases A, B, and C) are connected. Now, I'm not 100% sure, but I believe that both たり here mean that the situation keeps developing. So it's something like "if we have A, then (due to A) happens B, then (due to A and B) happens C". So if you can understand what A, B and C mean separately, you will be able to figure out the whole sentence now:
However, opinions like "A causes B and then C" also exist.
Do you need help understanding these parts too?