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Sure but they can definitely allocate time to have less homework. They do have some flexibility in their schedule as well, but I think the worst is some teacher’s laziness in grading assignments or updating grades. 
I’ve had assignments that we’d submit at the beginning of the semester and weren’t graded until the end. It’s difficult to know what grade you need to get on the final or how you should plan your final days of school when you have absolutely no idea what your grade is. 
It’s part of their job to grade, and hearing them complain about them having to do so is downright annoying.
I agree, if a high-school class requires hours and hours of homework a night, they need an overhaul of their curriculum or systems of teaching. Work smarter, not harder. >

Exactly. I especially hate it when you haven’t had a big assignment in a while and a break is just about to come and the teacher assigns a massive project over the break. They’ll be able to hang out and enjoy their time off while you’re working your butt off to get that huge project in before the due date the day you get back. Its infuriating about the amount of teachers who plan so poorly and respect your time so little >

Don’t blame teachers too much for it, they don’t choose the curriculum, they often don’t have much choice in things like assignments and work load.