Some teachers are just not that good at teaching.-----This is not grammatically wrong. However, I would suggest you to write this sentence differently to give more emphasis to your point. For example; ;
Some teachers lack teaching abilities.
You know, I have to say... I like the first way a little more! I like how blunt it sounds: "Some teachers are not that good at teaching."
It's like saying, "Some clowns are not that funny!"
Anyway, I also wanted to say, Dumi, that I wish we had some kind of cash prize for the most consistent, helpful contributors. We have no prize like that, though, so please accept my gratitude instead!! Let me know if you ever need a professional reference, letter of recommendation, or something like that. :-)
Because of this, some students lose their eagerness and motivation to learn because they know they can pass even without studying if they just pay.
Jerome, this can be a very good sentence. Like Dumi said, though, if you are talking about that bad teacher you had, you should write in the past tense.
*** Verb tense is the hardest part of English, I think!!!
:-)