I'm guessing 1) they already know they could get hurt doing that; 2) half of them have been hurt before from tripping while running, or being shoved into things while wrestling; and 3) they don't care.
Statistics on accidents while running at school might help, if you can find them. Also reminding them they might not be able to play in the big game on Friday (or run in the meet, or whatever) if they get hurt. And that it's one thing if they get hurt fooling around, but if they accidentally hurt someone else, it will be a big deal.
You still have to say it, just don't be surprised if they don't seem to listen.
True story: My daughter was running in the hall at school when a teacher spoke to her sharply. Still running, she looked over her shoulder to acknowledge the teacher and misjudged her distance to the steps in front of her. She slammed her foot into them, tripped and caught herself, then ran up the stairs. The teacher rolled her eyes and sighed and went back into the classroom.
We discovered the next day that my daughter had broken her toe.
My daughter still runs in the hallway.