CASify
How to do it
First switch your TI-Nspire CX II CAS off properly, with ctrl and on. This matters more than it sounds. A calculator you just put away is asleep, not off. In that state the key combination does nothing, and you end up trying for minutes.
Then hold down esc and also press on. Keep both keys pressed until the Press-to-Test screen is really there. That takes about two seconds and feels longer than you'd expect. If you let go too early, the calculator just starts up normally.
On the screen that appears, all restrictions are on by default. Use tab to move through the list, and press the center of the touchpad to turn single items on or off. The CAS model also has a switch for CAS itself. Your school decides whether it has to be on or off, and that's not a small thing. With CAS switched off, you lose exactly the features the calculator was bought for.
Confirm with enter. The calculator restarts and then runs in Press-to-Test mode.
Before you activate it, take a moment to think about whether you still need any of your documents. You can't get to them in Press-to-Test mode, and the only way out is with a second device.
What the mode actually does
In a test, a CAS calculator can do a lot more than is allowed. It has storage for documents, you can write your own programs, and the Notes app can hold entire formula sheets. All of that would be an advantage that others don't have.
Press-to-Test mode switches these things off for a while. The teacher decides exactly what gets blocked when the mode is turned on, so it differs from school to school. Some teachers give you a test code. Then everyone in the class is guaranteed to have the same settings.
While the mode is on, an LED on the top of the case blinks, even when the calculator is switched off. If it blinks green, all default restrictions are on. If it blinks red, someone has turned one of them off. That lets your teacher see from across the room whether your calculator is set up like everyone else's.
If the screen doesn't appear
Almost always, the calculator wasn't really off. Start over, this time with ctrl and on, and wait a moment before you press the key combination.
If you can't find Press-to-Test at all, check the operating system version. CX II calculators need version 5.0 or later.