I'm not sure where I heard this trick from but I used it on a grade 6 class and it worked a treat.

Before school starts draw a circle on the board that represents 1 hour. (can be adapted for different lengths of time obviously)

Label each quater with 15 min intervals and colour in the first 30 mins like so:

This clock represents 'negoitated curriculum time' (free choice activity time in the old language).

As the children behave well - work quietly, show manners and kindness, think creatively etc etc they can earn minutes of free choice time.

Why start with 30 mins already? so they can loose time as a penalty - for talking, or rudeness, or misbehaving etc right from the beginning of the day. Be sure the class knows why they lost time and they see you take it away. (and see themselves earn it back again too!)

I found this works really well but you have to be careful not be too generous or too frugal with points otherwise you'll run out of reward time by midday or the kids will be disheartened and loose motivation.

I wouldn't recommend this for students under middle primary - although you could try. Hope you find it helpful!