Paper
Everybody uses paper. It's a necessary component for human civilization and we can't do much without it. Unfortunately, paper is also one of those things that could easily be reused or reduced, but people still choose to recycle it for every single small mistake they make. Hopefully if you've read through the other pages on this website, you categorize these people as idiots (hopefully).
What's even more concerning is scratch paper. Scratch paper is one of the top reasons paper gets wasted. You grab 10 sheets of paper for a hard exam, but it turns out to be easy, and you only use 2 sheets. Unfortunately (but fairly), proctors must collect any unused scratch paper in most cases (who knows, you could have written something in invisible ink). Which, of course, must be recycled.
Other than that, it's often handouts that students don't really need. As a 7th grader myself, posting material on Google Classroom is enough, especially when it's something to refer to. The teacher should just let the students know that they can ask for a handout only if they need it.
Another thing I find annoying is NOTEBOOKS.
Material should be posted on Google Classroom, limiting the need to run through an excessive amount of notebooks.
Children who throw away notebooks.
If a notebook is unused, just keep the last 10-15 pages. You will eventually save a tree. Just use it for scratch paper whenever you need it instead of busting out new sheets of paper every time you need to add fractions or solve quadratics. Better yet, you can use your iPad as a notebook by using the new Freeform app.
If you really need a notebook, try to find a 70-sheet one rather than a 100-sheet one, especially if you notice from previous years that a big portion of a notebook remains unused even after the whole year.