Since students' essay writing abilities vary so widely, I'm always looking for ways to close the gap. One method I've tried was to have students collaborate to pull each other up. I've tried many different iterations of this - many of which were failures. However, one form has worked for me. I don't always do it, but if I do it once or twice a year, it spices things up and seems to help most of the kids.
I do this with an in-class writing prompt that should take the class period. Before class, I make two lists dividing students by their writing abilities. After going over the prompt, I call the names of the students, having the two groups stand at opposite ends of the classroom. In a barn dance style, students can find a partner at the other side of the room. This way, they have some level of choice while still being paired with someone of complementary abilities.
Then the two students write one essay; however, they make two identical copies of the essay - each writing their own as they go. When they are finished, they staple the two copies of the essay together with one rubric. A bonus is that I only have to grade one!
P.S. Don't have them work in three's - doesn't work...