No ones ever "proven" how gravity works, it too is a theory, as is atomic theory.
Remember, Newton's "Laws" of motion got corrected by Einstein's "Theory" of relativity.
About gravity, kids can prove it exists by dropping stuff on the floor. So, it is a proven fact of physics suitable for schools, while relativity, string theory and so on might not be suitable. About evolution, no similar experiment is there to prove the existence of evolution, never mind its theoretical mechanisms. So keep this stuff off the schools.