Cover all of the flooring and furniture underneath and near the water-stained tile to protect them from the bleach.
Put on protective eye wear and rubber gloves to shield you from the bleach. It's also helpful to wear old clothes that you don't mind getting stained with bleach.
Mix 1/2 cup of bleach with 2 quarts of water in a bucket.
Dip a sponge into the bleach solution and wring it out until it's no longer dripping, but still soaked.
Dab at the water stains, rewetting the sponge with the bleach solution as necessary. Do not rub the spot, as this may damage the ceiling tile.
Wait for the ceiling tile to dry, and then inspect it. Repeat the cleaning process if necessary. If the water spot remains, consider repainting the ceiling tile to cover it.