There are two issues here. 1. You think its a waste of time to do assignments you won't learn anything from. 2. You don't think the assignments should be part of your grade.
For the first point, remember that Roark had a specific goal in mind, and was only learning what he needed to achieve that. At high school level you are still learning general knowledge and thinking skills which should be applicable to most any career, so your work probably is contributing to your future.
For the second point, I suspect the teachers know that the students help each other with assignments and look at last years model answer for the same class etc. So they expect that students will all get the same high grade. So even though the assignments do count towards your final grade, they are not the differentiating element - the exam will still be that. So even if people are cheating don't let it bother you, just turn in the best you can for the assigments and know that you will beat them in the exam.