In HTML, what does the % (percent sign) instruct?

We recently “upgraded” to a new teaching program. Of course there are a million bugs that need to be ironed out, and we, in the fox holes, have to do it on the fly. One of those glitches, we figured out, is that when a student enters an essay and, for whatever reason, has a % (sign) in the essay, then submits it, it comes out blank at our (the teacher’s) side. So, of course, we reassign it thinking that the student just submitted the lesson trying to get out of doing the essay, hoping we wouldn’t notice. So then, of course, the student gets upset.

OK, computer Geeks…what’s up with the % sign?