From Quora: My answer to "How is the ACT company going to manage with students around the country scheduled to take the ACT over 2 days? Isn't there a concern of sharing the questions?"

Of course, there’s always a concern that students will cheat, share answers, copy questions, etc. If the ACT is giving tests over 2 days, they can easily use two (or more) different ACT tests. Teachers/professors/instructors have been doing this for a LONG time - a really good teacher will give a different test to a student who missed the original test. The ACT and SAT are standardized tests, which means they’re designed to be equally difficult. In an ideal world (at least for the ACT and SAT people), it wouldn’t matter how many times the student took the test, or what test version the student took, the score would always be the same. In the real world, they come fairly close. Discrepancies are usually from explainable factors (the kid studied between tests and improved his or her score; the kid was sick one test day and not the other; or the kid cheated- see below for a discussion of how discrepancies are handled).

I’m not saying that’s what the ACT will do, but it’s likely. Additionally, the ACT people probably do what the SAT people at the College Board do - they cross-check scores against reported school grades, etc., so “outliers” (e.g., a D student who gets a near-perfect score) can be investigated. Sometimes such people are asked to re-take the test; sometimes the matter goes to arbitration; sometimes the student and test company agree to have the score canceled.

Don’t worry - the ACT people most likely have things under control - very few people are going to get an unfair advantage on the test.

Click here to see the original answer on Quora.