Where can you see a grade after a match ?

It's on top next to your IP meter. If you mean like in your match history, not in the post-game lobby, it isn't displayed anywhere. You can see your highest earned grade for each champion in your champion page of your profile though.

You can only see your highest champion grade by going to champions page in LoL client in your profile and sorting it to "Highest Champion Mastery" and hover over the champion

I would seriously like to see this also. I accidentally clicked play again before I saw my rating on a new build experiment.