I've had a traditional "first day" promise I've given my students for years.
We start with a nice talk about why we have school. The children have no trouble coming up with "we're here to learn".
They know it's their responsibility to learn.
My "first day" promised is based on that point. I always say the promise exactly the same way each time I say it, with the same gestures.
Later in the year, if I do these same gestures, they are reminded they are here to learn, and I don't even have to remind them aloud.
Here's the promise, with my gestures:
We are here to learn. As long as you do your best to learn, (Move right hand across body.)
I'll do my best to make it fun. (Move left hand across body.)
That's it! It's simple!
Whenever the little ones get a little silly or unfocused, I remind them about the promise. They manage to pull themselves together and show their learning behavior.
Of course I manage to keep the "make it fun" part of the promise. I like the fun, too!