Yes, that's how long term memory works in the brain. Explaining things and social learning are excellent learning techniques, I'm not denying that. These days smart phones and social media are an excellent tool for communicating ideas with your peers. But it has to happen with peers that are on the same level as you are. Otherwise you are explaining too simple things that you've already mastered.

This "discarding after test" is already and automatically addressed in the education: high school repeats things learned in previous schools and builds on those. Our high schools also have large final exams that require one to be able to understand the whole curriculum of the subject. The grade measures your memory, and understanding because you cannot just memorise everything. The same is true for all education levels.