"Ustedes han demostrado que pueden correr."
Translation:You have demonstrated that you can run.
I think your teacher is smart to teach it that way because it reinforces the meaning & usage of ustedes. But when you are speaking English to your three friends, don't you say Do you want to get together and study tomorrow? You probably would not say Do you all want to get together...? Most of the time American English speakers drop the all when addressing several people.