@chorean, actually there is a button somewhere that says something like add another school. As I do always err on the side of caution even if it’s overkill (an ounce of prevention is worth a pound of cure), I would still email them. They save every correspondence within the application so everyone can see it.
It says all over the place to list all schools and not doing so could jeopardize your admission. While high school is probably not as do or die as forgetting colleges, the omission is still against what they state.
Let’s say you didn’t list high school and you updated, as you did. Probably 90% it won’t matter. (I’m guessing, how do I know?)
However, you send a message saying you just found out on College Confidential you were supposed to list all, you thought just graduation, it suddenly won’t matter 100%, and everyone will see it.