From the Mishna B'rura's introduction to the rules of Shabas:
…But by what means can we arrive at this level: that one keeps Shabas in all its details? The advice on this is that one urge himself to study the rules of Shabas and to review them always so he knows what's forbidden and what's permitted. Without that, even if he learns all the exhortatory things that urge one to keep Shabas properly, that won't help him.… [Rabbi Yonasan Eybeschutz] already assured us that it is utterly impossible in practice to be saved from committing a Shabas prohibition unless he learns all the rules very well.
I seem to recall a similar idea written about the rules of lashon hara — and, indeed, much the same can be said about many areas of halacha.
Considering that a twelve- or thirteen-year-old is obliged to follow all the details of halacha, it would seem, therefore, to behoove elementary-school teachers to focus on halacha to the extent that their students will retain it. Yet we don't find teachers doing so: halacha is relegated to perhaps four hours a week. Why is this?