I would look to move to a different school, in your position. We have in fact, moved school thrice for a combination of reasons and my son is in 2nd grade now.
As for the "show your work" thing: my DS is very strong in mental math and it does not make sense to him to show work where the answer is very obvious. He got 15 points taken off his last math test because he wrote the final answers and did not show his work - for obvious questions like "Timmy has $40 and wants to buy a book that cost $12 and a toy for $23 - can he afford them?". He is so used to scoring well in math that the points taken off mattered a lot to him.
We are using his homework sheets to brainstorm how to show his work - we use number lines, write descriptive sentences or even pictures to illustrate the "show your work" thing. It is not ideal, but, my DS is beginning to realize that his math scores depend on these things and he is willing to work at it now.
