Do not wait until the student has gone too far and has to recopy too much. At the beginning, as they write slowly, the students will hold their pencils too tightly. They will complain of pinched fingers. Tell them that this will pass quickly as they become more relaxed and sure of themselves. Do not make an issue of it.
Many students hold their pencils in grotesque positions.
These students can be taught to change those positions at any grade level if a teacher is persistent in requiring the change. The change should come in first and second grades. In the upper grades the students' bad habits are so firmly established that you may become discouraged. As the students' writing improves, post a sample of their new writing under their autobiographies. Let the students see the contrast.