Whether the written response has high stakes (such as a college application or SAT/ACT essay) or is part of an ordinary school task (such as an open-ended question or literature-based prompt), there is one common mistake students make that has a straightforward solution: Answer all parts of the question and provide proof!
A successful response must include the following: direct answers to all questions asked, well-developed support, references to relevant examples/illustrations, and clear explanations of how the support and examples prove the answer.
Often, writers focus so much attention on the writing process itself that their responses wander off topic or fail to answer all parts of multiple-question prompts. This is why writing has four steps: brainstorming, prewriting (organizing), writing, and rewriting (editing/proofreading).
Adults, take note! The above pointers also pertain to work-related tasks and projects.