The civil war was not fought over slavery. It wasn't even fought over keeping the south in the union. It was fought to keep Brittan from reconquering the US.
The south didn't have any money. Slavery in the south made commodity traders in the north rich, not the slave owners. You may find that hard to believe... How can you own slaves and not be rich? How can you live in one of those huge plantation houses, and not be rich? The economics of slavery favor the slave trader, not the slave owner. And those plantation houses look huge until you realize it housed an extended family of 20-30 people, plus house slaves. There were white people working the fields right next to the slaves (and they were treated only marginally better).
The south didn't have any money, but war is expensive. So how did the civil war even happen?! Turns out the south had a friend across the ocean willing to lend them very very large amounts of money. Now what could the UK possibly want in return for funding a civl war? America split in two, that's what. Divide and conquer. The war of 1812 was only 50 years ago, and Britain had not yet given up aspirations of reconquest.
Lincoln didn't free the slaves because he's a nice guy. Lincoln proclaimed emancipation to make the British government's support of slave-owning confederates EXTREMELY unpopular with the British people, who were vehemently abolitionist. Lincoln turned a war about the economic oppression of the south into a war about slavery, and in doing so, isolated the south from the rest of the world. Without the support of the UK, or the industrial capacity of the north, the confederacy was doomed.
They don't this in schools because anyone who says the civil was wasn't about slavery is a racist confederacy apologist. The fact that you don't know the civil was was about keeping North America free of the tyranny of the British crown is DANGEROUS..... and the political correctness that lead to that ignorance is one of the tumors slowly killing America.