Actually, the tallest mountain on Earth is not the highest: http://apod.nasa.gov/apod/ap160225.html
The use of "tall" here is not preferred, and is inconsistent with other sentences on Duolingo. However, in common usage it will definitely be understood, and so it should continue to be accepted (along with "high").
I think that even in England if you were talking about high mountains and were going to use the word high a couple of dozen times, you would start looking for acceptable synonyms. The most common one in English, when talking about mountains, is tall. Greatest elevation is sometimes used.
These synonyms are particularly useful when trying to distinguish whether the height is being measured from from the center of the earth or from the surface since it makes a big difference. Which is to refer to the difference between how far it sticks up into the atmosphere and how far you have to climb.
Apparently you restrict yourself to speaking of mountains as being high. Others sometimes speak of them as being tall and for good reason.
A high mountain refers to how high it protrudes into the atmosphere above sea level. A tall mountain refers to how much it protrudes from the surrounding terrain. The difference may be thousands of feet.