For me, the answer is simple, EVERY project must be tied to its maker. If I am forced to make something I have no connection to, then it will be pretty but hollow. It's not that I don't care, it's that I am not connected. In a classroom setting, this would be levels "B" or "C." Students without an affinity for art will more likely fall into the "D" or even "F" category out of defiance.
I have a whole blog post detailing the importance of connections HERE, but the gist of it is that with a little effort every art exploration can be personalized. I have 7 methods HERE that I use to be sure the explorations I design allow for individuality and can be used from kinder classes through advanced art courses.
When explorations are personally connected, students care more about their work product. "Because I said so," just does cut it. I hate when people say that, and I am sure my students don't want to hear it from me... ever.
There will be days, even with connected explorations, that students will be less engaged. These are opportunities to sit with them and have a chat. Maybe they have hit a mental hurdle. Maybe they have taken on something beyond their skill-level. This is where feedback and suggestions can be helpful. These can come from you, but the feedback can also come from peers. Sometimes a mid-project critique can be helpful, while there is still time to shift gears or make adjustments.
Students need to be working every class period. They need to be engaged in the process of creating. It's part of my rubric, and I grade their participation daily, but not in the way you might think. My first grade every quarter is "Studio Habits." It encompasses a lot, from setting up your work area to cleaning up after yourself, as well as being on task for the bulk of your class time. More about that HERE in detail. I also have my Universal Rubric there so you can down load it.
Additionally, I do not allow students to take work home because I cannot verify if the work is truly theirs. This also avoids the issue of a student choosing not to participate and saying, "I'll just do it at home." As a new teacher I fell for that, but changed quickly when work never came in or was obviously done by someone else. There is no substitute for creating art under the watchful eye of a knowledgeable certified teacher.
My policy is that I grade all work on the due date, and then students can take work home, and even do more to it. I allow students to return their work for a higher grade should they put in the extra effort. I do not make deductions for lateness. This means if an assignment is 50% complete, I will record a 50% as a grade. If it comes back, before grades close, and deserved a 95%, then I overwrite the grade accordingly.
If you like the poster above, I have that and more on my Zazzle page HERE. I suggest you wait till they run a sale for better prices.