What should a student put in the additional information section of a college application?
Don’t use that section unless you need to. It’s not required.
You put things that you couldn’t put elsewhere in your application.
Usually it’s anything significant that caused your grades to plummet, your EC’s to be weak etc. This can be illness (missed school), severe depression, parent’s divorce, death in the family, family responsibilities (had to take care of a sibling because parents worked long nights, had to work to support family…) etc.
Maybe during 8th grade you were in Korea and moved here to the US during 9th grade, things like that.
So if nothing extremely like that happened during your 4 years of high school, then don’t put anything in it. Not writing anything in that section won’t hurt your app in any way.