Exercise #1 - Think before you do something
Sometimes once you are done something you might stuff it in your desk or the wrong place in your pocket folders / portfolio and then you lose things. The few extra seconds it might take to put the paper in the correct spot could save you the time to do the assignment all over again. It may seem quicker to just stuff something in your desk but there is also the time to take it out which is a lot shorter if you are organized.
Exercise #2 - Plan ahead
You can't be organized if there isn't a place where things are supposed to go. Dedicate 40 minutes or less to cleaning your desk, backpack, locker, and folders / portfolio, and it will make life a lot easier for you, as now it is much simpler to find the things that you have for a certain subject, such as finding a booklet for science. At least when you'll be looking for something, you can be assured that you'll have whatever you were looking for can be found.
Exercise #3 Create a System
Different people have different ways of doing things, and that applies here as well for staying organized as we have different ways of keeping our stuff where we know it will be in our binders, such as color coding it. However, whatever works for you, is your way of being clean and able to find your stuff easily, so just form your own system.