As the lab TA below describes, it is a good idea to use multiple methods of communication with your students and to repeat important due dates and messages. This is not only helpful to your students---it can provide important documentation should a student claim they were not aware of what was required of them:
I had a student who would not do his assignments, with the excuse that he did not have internet at home and so could not check his blackboard or read my emails (which is a pretty bad excuse in itself). He then accused me AND the university of making it difficult for him to learn. However, because I had made it a habit to give all the instructions in the lab, to remind my students of the assignments that were due, and to then send them an email reminder of all that we had talked about in the lab, I was easily able to counter his accusation. My suggestion is to try your best to have all your grounds covered before you enter the lab or classroom, and to keep them covered over the course of the semester.