I use a mini stop sign when I am assessing. I keep it at the table where I am working, and hold it up when I see another child approaching out of the corner of my eye. It's a gentle reminder, that I am assessing and cannot be interrupted.
Make another one, laminate it and tape it to a large Popsicle stick. Quietly hold it up when you want students to stop what they are doing and listen to a direction.
Time them to see how long it takes for everyone to come to attention and raise their hand (Give you five.) If they do it within a certain amount of time (60 seconds) they receive stickers, or a scoop of popcorn seeds in the popcorn jar, a letter to spell a certain treat etc.
The stop sign can also be used when students are playing games.