what was the outcome? Did they come back with arguments the next day?
"Many people believe things based only on what they hear other people say, not based on real evidence. This makes it easy for people to have misconceptions (wrong beliefs). One common misconception is that the Earth is a sphere. The majority of Americans believe this in spite of the evidence to the contrary.
"The Earth is actually a disc (circle). The sky is a dome above the Earth. The sun, moon, and stars are all attached to this dome. The dome rotates, giving us day and night. Underneath the Earth there is water. Around the edges of the Earth is a vast, impenetrable ice sheet. The north pole is at the center of the Earth.
...
"Point #7: The United Nations’ logo correctly shows the shape of the Earth. Since they are the supreme governing body of the Earth, they should know how it is shaped!
...
"Point #9: This one you can try yourself! If the Earth is a sphere, then the ground must be slightly curved. This means that the floors of buildings must be slightly curved. Lay a ball or a pen on the floor. Does it roll?
"Point #10: If the Earth is a spinning sphere, then all of the water in the oceans would be flung into space!
...So you get the point. If I had more time, I would have made the lesson more compelling.
Most students exhibited some form of cognitive disequilibrium. About half ended up going along with the concept of a flat earth. About half were skeptical. A few, especially one student, got quite interested and came back at me the next day with lots of evidence and reasons to the contrary. So it had the intended result for at least a few students. And at the end of it all I think most got the point that we should rely on evidence, not simply what people say.
...well...yes and no. For basically all of our beliefs about things outside of our immediate environment, we rely on evidence that other people have collected. I can't determine that the earth is a sphere by my own observations any more than a man could 5 thousand years ago.
The way the stars and shadows change with even a small shift in latitude was a dead giveaway to men ~2500 years ago. Even if you never left home, you might notice that the masts of ships on the horizon come into view before the hulls. And on second thought, why would a horizon exist on a flat world anyway?
For this example at least, you certainly could rely on first-hand evidence. You might need people to put the pieces together for you, but you can make the raw observations yourself fairly easily.
If the radius of the Earth is r, and you have height h, then from the top of your head to the center of the Earth to the horizon and back to the top of your head is a right angle triangle. The hypotenuse is r+h, one side is r, and the other must be sqrt(2rh + h*h). Under the assumption that r is much bigger than h, that means that the distance to the horizon varies as the square root of the height.
This assertion is surprisingly easy to check with ships. Furthermore if you pay close attention on a commercial aircraft, you can actually see that the horizon is a little bit below level!
Still, there are empirical studies on how people, when presented with information of the form "not X", "X is a myth", etc, will forget the "not" and "is a myth" parts of the information first and thus be left more ill-informed than had they never been presented the information (I'm in the middle of packing, so no link, sorry; but I seem to recall something about a "Myths about the Flu" pamphlet in one such study). Other studies address people's fallibility when it comes to usefully remembering where they learned particular facts. So if you are going to keep doing such lessons it might be helpful to look into such research and perhaps try to assess later just how your lessons had done, aging-well or breaking-down over time, in memory.