Ah, sage advice from an octogenarian.
My school employs a team of senior citizens to proctor exams. My section has the same 4-5 proctors for all of our exams. One of these gentlemen, John, is the most dependable of proctors. I've heard him make the instructional speech several times, and he's the same every single time.
He gives us the same advice every time:
"You need a good eraser for Scantron sheets. If you don't have a good eraser, the machine might not read your answers right. You're spending $30 thousand on your tuition. Go buy a $0.25 eraser."
"$37 thousand," I mutter under my breath as my fingers grip my awesome Japanese eraser in pre-test anxiety.