My bleaching preference is a paste created from magnesium carbonate and 50% hydrogen peroxide. Mix the chemicals together until you get a glue like consistency. Paint the paste on and place the skulls in the sun for a day. Each chemical can be purchased over the counter from chemical suppliers or commercial cleaning chemical suppliers.
As a daytime chemistry teacher/nighttime taxidermist, please use caution when working with chemicals. If you choose to go this route be sure to mix chemicals in a well ventilated area and wear protective clothing (eye protection, gloves, apron, long sleeves). This mixture initially produces fumes and can provide chemical burns if in contact with skin.