Just to give you a sense of how many you can miss and still get a good score, I just checked my ACT book.

To get 25's you would need to get the following number right:
English 55 out of 75
Math 40 out of 60
Reading 25 out of 40
Science 29 out of 40

These vary a bit from test to test, but you can see that you really don't have to get them all finished to do well as long as you do well on the ones you answer. But there is no penalty for guessing so it it worth it to fill in on the bubbles even if you don't finish. That what's so great about these tests for most of our kids, finally lots ceiling.

To get scores in the 30's you'd have to finish and I know there are kids who do finish. But there is no shame in not finishing.