The premiums are not 'over the top' Coccinelle. Unfortunately, the premium is based on your sons being considered a particularly high risk. They have no experience (it takes about 15 years to be regarded as an experienced driver) and their are many other factors to consider such as their age, occupation, address etc.
You have to remember that it is the insurance company who are the ones who will have to spend time and pay out money if your sons have an accident. The premium therefore reflects the risk to the company.