One of the biggest problems faced by students when writing is that they feel overwhelmed. Short assignments with which students feel comfortable are always preferable to long essays. Remember the level of the student. The ACTFL Guidelines will help you here. Remember also that the goal should be communication and not just perfect grammar. Most people do not communicate perfectly in L1. How then can students be expected to communicate perfectly in L2? If we dwell on the errors, the student with special needs may be inhibited from developing written skills.