I think that If I was taking over from you, I would really appreciate being left a sheet of paper with a quick sentence or two on each student. It will only take half an hour if you spend a minute on each child. I appreciate that it's not technically your job if it occurs after you leave, but imagine how much harder and time consuming it will be for the next teacher than it will be for you. Imagine also that teacher's look of gratitude! Plus, isn't it unfair for children that have worked really hard / been really naughty for you to have no mention of that on their report, just a vague comment from someone who barely knows them?