It's sounds as though your teacher has committed historical malpractice in trivializing something as complicated as the First World War.

that it was because of money.

That's a very odd explanation for the cause of WWI considering that that mere outbreak of the war set off a world-wide economic collapse.

The theories that were being circulated before the war, perhaps the most prominent and best known being Ivan Bloch's book, published in English as Is War Now Impossible? which argued, among other things, that the scale of industrialization and globalization meant that a major European war would result in a financial armageddon so great as to, as the title suggests, make it impossible.

The French socialist leader Jean Jaures is quoted as saying before the war that "international movement of capital is the biggest single guarantor of peace."

Both the German and the British war planners were devising their war plans on the basis that war had to be swift because it would be economically devastating (for different reasons, the Germans had to accomplish the Schlieffen plan and end the war quickly because their economy could not sustain it, the British because any European aggressor would be destroyed by Britain's dominance in international capital and trade.) That was one reason why so many people were fooled into believing that the war would be "over by Christmas."

For more on that I would look to, for example, Nicholas Lambert's Planning Armageddon or Niall Ferguson's The Pity of War.

What was WW1 really about? ... we never learned about the real cause

The the outbreak of WWI is the one of the most complex events in human history. If I recall correctly, I think at one point in his book The Sleepwalkers Christopher Clark claims that it is the most complex event in history. I personally am inclined to agree with it.

The assassination of Archduke Franz Ferdinand is the obvious proximate cause of the war, but it is entirely inadequate to have been the sufficient cause of the war.

Historians can and do write multi-hundred page books examining the causes of the first world war. It is an unsettled question and one that our ideas and understandings of have changed repeatedly over the course of the past century.

As an introduction I think Michael Neiberg's lectures here and here. The first one in particular explains why the simple explanations for this that are typically taught at a school level just don't make sense.

There's a virtually unlimited set of literature that I could point you to. As I mentioned I think Christopher Clark's the sleepwalkers is quite good.

But to summarize why the question is so complex that Clark lays out I think it's worth breaking down is entailed in the question "What caused WWI?" Because what we mean by that, at a very basic and preliminary level is, among other potential questions:

1. How did these alliance structures form?
2. How did a Balkan crisis lead to a general European war?
3. How did each country interact with the crisis that preceded that conflict and then how did they enter into it?
4. When the war broke out, why and how did it then take on the character that it, of which the most notable aspects are arguably its carnage, futility, and stalemate.
5. Who was making the decisions to do this and what were their motivations for doing so?
6. What were those decisions? What impact did they have?
7. Was someone to blame? Is it worth assigning blame?

And so on and so on. And mind you, all of these need to be answered in about a dozen languages for the respective parties.

The last point I think gets to why it is so difficult to answer. With a war like WWII, circumstantial causes notwithstanding, we have very obvious parties to blame. Hitler is a pretty easy blameworthy figure.

For WWI while many such potential villains have been identified (Germany in the Fischer thesis. Britain in some senses in Ferguson's book. Russia in Sean McMeekin's *The Russian Origins of the First World War.) But none are entirely convincing and there is always a "but also...".

Anyways, happy to take a stab at followup questions but I hope that points you in a good direction.