[dee]

you wrote: "I then see students who were still start to move just to get my attention."

Bingo!

Ring the bell, praise only those who are freezing (immediate verbal, then a surprise reward perhaps).

Second graders will do what they can for attention, good or bad, and if you only say Johhny, please freeze, that's reinforcing attention for a behavior you don't want. Mary, thank you for freezing should result in more students freezing.

Hope this helps!

When I taught first, instead of writing "bad" kids on the board, I write "good" kids on the board. You will not believe the difference this made in my classroom!