A tiny bit of the sand on the beaches comes from the eroding sea cliffs, but the vast vast majority of the sand on the beaches in washed into the sea from rivers.

Mountains and hills are eroded by wind and water and all the bits of rock -- from tiny clay particles to sand to big boulders -- ends up in the rivers. The rivers break up the boulders into more and sand and clay and carry all these small rock pieces out into the ocean. The clay gets washed out into the deep water, but the sand-sized pieces are heavy enough, that they fall down to the bottom as soon as the rivers enter the ocean.

Ocean currents called long-shore currents carry the sand away from the mouth of the river and down the shore. In California, the long-shore currents move southwards. Sometimes you can notice them when you swim in the ocean. You will find that you are drifting away from the spot you entered the water. The sand is moving in the same direction, but we don't usually notice.

A really good 20-minute movie on this subject is called "Beach, River of Sand". Your school district may own a copy you could borrow.

watch here.