Norms govern the criteria by which students decide what is good and what is not good, and align their learning trajectories accordingly.We found that the high-school students' norm is to produce working, but not necessarily error-free, programs and to argue for their correctness solely on the basis of a few executions. Therefore, they prefer working directly with the computer, refining an initial idea iteratively by trial and error. To change this norm, instruction should provide a fertile environment for the growth of professional norms as an integral part of computer science training. Specifically, we suggest integrating explanatory proofs as a means of establishing norms of precision in the students' practices and methods of communication. In order to demonstrate how our approach is implemented, we describe a case study, portraying the teacher's role as a norms facilitator and illustrating the students' adjustment to the instructional activities. (Contains 2 tables.)