I'd recommended Ankidroid flashcards with one of many kanji learning packs, pick one with good reviews. Mine isn't separated by grades but help me a lot to memorize general must to learn kanjis.
I bought this from my local book store, and its organizes the characters by grade level from 1st till 7th I think, *I haven't gotten past first* but I do remember it was past 5th, till it reaches the adult. Its not free but it has good ways to remember them as well. Here's a link, I used to go to my local books store and just study it while I was in there.