<p>I just spoke with College Board about something else and asked them specifically: if you are registered for 2 or 3 subject tests, you can either switch the subject of any of those tests on test day at the test center, like aviatrix10 said, or drop any of them from your schedule that day: you just leave after the last test and don’t take the other one(s). The only time there is an extra charge is if you want to ADD one or more tests to your schedule.</p>
<p>So in your case nobodyjay, you would just sit, take bio, take math, and leave after that.</p>