I see this question posed frequently by elementary, early-childhood, and general music teachers, in different forms: "How do I fill up the time?" I think this question comes from thinking about music education as a series of activities. Activities in the music classroom seem to be a model favored by our profession, lots of the curriculum, lots of lesson plans, encouraged in music teacher preparation programs, etc. Many teachers and curriculum designers strive to develop activities that touch on music concepts or ideas or standards, in an introductory or exploratory way. And so we need activities to fill up the time.
If we're struggling to figure out how to fill up the time, I think a goal-oriented or goal-driven approach can help. One where we set learning objectives and strive to ensure each student achieves those learning objectives. Where we're not simply touching on topics but practicing them and learning more deeply so that students actually develop real musical knowledge and skills, that can then be put into practice in performance. If we sit down and make a list of all the things we wished we could teach a grade level in a year, so that each student became proficient in a grade-level of musicianship, we would quickly see that there is no where near enough time to accomplish this, in whatever time we had -- even if we had two 90-minute weekly classes, for example. But doing this also will demonstrate how easy it is to fill up time: with teaching and developing real usable, presentable skills. There won't be enough of it to accomplish all we want to accomplish.
If we move away from the introductory, exploratory, activity-based model (which often feels more like entertaining kids rather than teaching music to students) and strive to teach real musicianship with the goal to develop real musical skills in all students, we definitely will have the problem of not having enough time! But we won't have the problem of trying to figure out how to fill up the time!