It won't scratch. I've had dirt get caught in some of my screen protector installs, and when I decided to take it off and install a new one later on, I didn't see any scratches on my screen at all. This applies to both my iPhone and iPad.
Although, on the iPad, it's a bit harder to do a perfect install of them. But you shouldn't have any worries. 😉
That's never happened to me on any of my devices. I know my wife had the invisibleshield and some cheap rubber/plastic case that would lift it up, sand got trapped under it and she didn't care to clean it. I'm not just talking about 1 or 2, it was like she dropped it in the sand and had like 20 sand particules under it. I was afraid it would scratch it but to my surprise it didn't.