A Scantron is the brand name (I think) for those machines that process tests with the little oval "bubbles" that you fill in for standardized testing, so that it can be graded by computer. They have to be filled in fairly neatly where you fill in most of the bubble but not too much can go past the oval and you can't fill it in too lightly. If your kid writes too hard they can rip the page, too. Erasures have to be done fairly well, etc.
It probably depends on the rules of the test itself what modifications you can get for standardized tests. If the answers are onsite at the school, and the rules allow it, often they will have the Special Ed teacher review the test later, to see if things were marked wrong by the computer because of those kinds of issues, and the teacher will mark anything right that was answered correctly and just graded wrong because of the scanning issues.
If it is a test where that is not allowed or they don't get the answers, I have no idea what they do, so you'd have to ask.