The arts tradition of exhibition is followed. If in the arts it was all practice, and no recital, no one would work hard, or care. But in most schools that is all children do-practice. If, on the other hand, everything they do will be exhibited, then they begin to care very much indeed. Finally, the arts' tradition of keeping work in a portfolio is followed.
Evidence fits scientific inquiry but fits well in the arts as well.
Ron's school is entirely arts infused and hasn't issued grades in 20 years. The kids in town love to go to school. The school is filled with art supplies. Everything the kids do is documented. Multiple drafts of work are expected, with critique sessions from fellow students.