In addressing conventions of grammar, punctuation or usage, distinguish between intermittent slips and patterns of error (indicating a lack of command) in students´ texts. How you respond to intermittent slips will likely depend on the stage of the draft and what kind of instructions or expectations you´ve established.
Risk-taking and error are absolutely essential to growth. If you identify a pattern or patterns of error, understand that they represent an effort at something rhetorically or linguistically new and difficult for the writer. This is not a careless or deficient writer but someone who is pushing him/herself in his or her writing. (Bill Strong, in "Coaching Writing Development," argues that good writing teachers learn to read for what students are attempting to accomplish syntactically and rhetorically.)
If a writer lacks command of a particular grammatical convention, research on language learning suggests that she is best poised to begin learning that convention: --when she is committed to a text, a purpose for that text, and an audience for whom getting that convention "right" really matters. --when she is at a stage, with that project, that allows her to focus on how individual sentences are working to achieve the larger goals she has for the text. --when the convention is presented to her in words she can understand in the spirit of helping her succeed in the goals of that text (Weaver).
If a student´s text suggests a lack of command over several conventions of grammar, punctuation or usage, work on one or two at a time. (You might prioritize which conventions to address on the basis of which patterns have the biggest impact on the effectiveness of the piece given the student´s purpose, or the student´s ability to develop the piece. Some researchers have argued that you should address higher-status conventions first. One set of studies found that professors were more likely to downgrade a text on the basis of a wider number of errors than business-executives−but that both were equally likely to identify as "serious" errors such as the following: fragments, run-on sentences, and mistakes with subject-verb agreement.)
Gaining command over a particular convention requires repeated, meaningful and contextualized engagement with the convention. Isolated drills (or ignoring the problem) won´t work.