Don't. Instead of putting forth the effort required to pull off these cheats, you could be studying. This will probably get better results, and won't put you at risk of ending your academic career.
Also remember that the teacher or proctor administering the test will be aware of these techniques.
The buddy system
Looking over someone's shoulder for an answer, or accidentally dropping a pencil to get a better view are as old as time, and probably won't be affective.
In some high school classes, you may have a reading assignment to do after the test. Have someone near you open the book and hold a cheat sheet onto it. If asked, it's just some left over notes they were writing on. If that seems risky, have your partner open to the chapter covered in the test so you can look over and read from it.
There are a few good ways to sneak in written information.
It's well known that cell phones are used to pass information. You can still use this method if you "accidentally" forget to turn off your cell phone. Have someone call or text you during the test. While you are trying to shut it off, you can quickly glance at your notes.
Graphing calculators can hold text as programs: type out your formulas and explanations, and view them when needed.
Another method is to alter the text of something that is in plain view. If you are allowed drinks, you can make a new label with your cheat sheet on it, like this:
This makes a label that from a distance looks like one from a regular drink bottle. Try using glossy paper to more closely match the real label. It won't fare well under close inspection, but from a distance it looks convincing
Do you know how to write in a language that uses non-roman characters, like Japanese or Chinese? Draw on a nicely-designed "tattoo" in this language containing your cheat sheet.
Information can also be written on disposable items, like gum wrappers. When people still smoked in public places, students would write answers on cigarettes and smoke them as they worked on the test.
Posted 5005 day ago