If you're a high school student (or the parent of a high school student), you probably thought that the ACT is just in the business of creating and administering the ACT test, right?
Actually, they do quite a bit more, and one of their "side" businesses can affect your college applications without you even knowing it. Please read the following advice carefully:
In order to increase its profitability and market share, the ACT has been developing other "predictor" tools to sell to colleges. Basically they slice and dice any information they have about you, the student/test taker, and they generate performance predictors beyond your ACT scores themselves. You can read more about that product here.
The key thing for you to understand is that the ACT will use all the information you provide when you either (1) register for the test or (2) request that the free test score reports be sent to your colleges. The ACT will factor all the grades, courses, and GPA information that you give them into this predictive tool, and it will transmit the results to participating schools.
The smart thing to do is not to disclose any of this information to ACT, the College Board (the people who make the SAT), or any other service that sells information to schools (for example, online college search or college matching tools), because doing so won't help you, but it can hurt you. Update: When you get to the page asking about your high school courses and grades, just leave those questions blank and hit the Continue button.
Here's an example how disclosing that information can undermine you. Imagine that you have solid ACT math scores, but your current math grades aren't so great, and you aspire to enroll at the business school of a college that subscribes to this predictive tool sold by the ACT. Your ACT predictor score will be lower than your ACT math score. That's because the formula that the ACT uses under the hood of that predictive tool will be applied to whatever information you happen to disclose at that particular moment in time. The results will be communicated to the college as a "constant" and will live in your student record forever, even if you subsequently improve your grades.
Bottom line: there is no upside to disclosing that additional information to the testing companies, but there is potential downside. Don't do it.