The short version is that but for slavery, there's no Civil War. We can unpack it a little and say the the immediate cause of the war is secession, but the cause of secession was slavery. The principals were extremely explicit about that when they explained themselves at the time, only changing their tune after losing the war. There are some other issues, but they're all essentially ways of expressing concern over slavery's future in the Union.

Likewise when someone wants to beat the old state's rights drum, the white South is remarkably consistent in caring about state's rights only if and when it appears that the federal government may do something to weaken slavery, whereas otherwise the same people are downright eager to force recalcitrant free states into submission to that same den of tyranny on the Potomac they otherwise castigate.

We have a great FAQ section on just this question. It includes a couple of answers by yours truly, but the real standout is /u/georgy_k_zhukov 's mega-post.

That said, there are a whole lot of moving parts here and the last thing any of us wants to do is discourage you from asking questions; they're what keep the sub going. If you can expand on your confusion then I know I'm not the only person happy to dig into specifics. Is it just the disagreement between your teachers, or is there an argument one or the other has made which you'd like some light shed on?

We're here to help. :)

The easiest solution to this question is to look at the documents each confederate state released explaining why they were trying to secede from the Union. Here's a good link: http://www.civilwar.org/education/history/primarysources/declarationofcauses.html?referrer=https://www.google.com/#

Check the second line of Mississippi's declaration: " Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world."