If the tag can't work as the only tag on a question, it's probably a meta-tag. Every tag you use should be able to work, more or less, as the only tag on a question. Meta-tags, like [beginner], [subjective], and [best-practices], are useless by themselves -- they tell you nothing at all about the content of the question.
A homework question can be about literally any facet of the Japanese language. For example, this question is tagged homework, but it appears to be about relative tense. It's not about homework, and knowing that it's from a homework assignment doesn't tell us anything useful about the content of the question.
This tag is only on eight questions, so the best solution is probably to edit each question by hand, removing the homework tag and adding whatever tags are appropriate in its place.