SOLUTION: My teacher told me that on a surface of a sphere, you can have a triangle with THREE right angles, is that true?

Yes.  This is usually stated as this riddle:

A hunter walked one kilometer due south from his camp. Then he walked one
kilometer due west. There he shot a bear. Then he walked one kilometer due
north, and found that he was back at his camp. What color was the bear?

Imagine yourself doing this: 

1. Start at the North Pole, go one kilometer due South.
2. Turn 90° and go one kilometer due East.
3. Turn 90° and go one kilometer due North.

You will end up at the North Pole and you will have traced out a 
spherical triangle with three 90° interior angles.

The answer to the riddle is "white" (a polar bear).

Edwin