I give a "completion" grade. I spot check them weekly and grade them based on whether or not they are actually writing (complete sentences, ideas, etc.) I do not mark grammar or worry about sentence structure, but the purpose is writing. If they are there and have 15 min, they should have a seveal sentences minimum, with thoughts about one subject (either the one that is in the prompt or one of their choosing). I do not accept random words, repeating the same sentence multiple times, or drawings, etc. I also teach fifth.