Hi Sara,
Only just picked up this thread as my computer has been away for repair.
Teenagers do sleep a lot; after all there is a lot of growing going on; between the ages of 11-14, they go through what is known as the "adolecent growth spurt", which is controlled when the horomoned Testosterone is released in puberty and Progesteon in girls.
Girls tend to hit the growth spurt first, when height, weight and the size of the pelvic girdle increases. In the latter cases this is due to a girls body preparing for the onset of mentuation and the ability to conceive. The provebial "puppy fat" is a layer created by the body so that the energy used to grow and maintain sufficient energy for the menstrual cycle to be effective and regular. If she were to become pregnant an energy store ( fat and carbohydrate) to feed the baby. Also for the body menstrual cycles take a lot of energy, and blood needs to be replaced by the spleen after the period.
A girl can leap in size (in both directions) up to three dress sizes in a year; in the same way as boys can jump from a 36" chest (sorry I am not a metric person) up to 40" and beyond in the space of two years, Also their shoulders grow wider than the chest and the muscles develop, in proportion with their height- hence growing pains as the body gets taller very quickly. In both cases this is all triggered by the hormones mentioned above and to a certain degree genetics since when grown up, both sexes tend to be withing 3" of their respective parents' sex. Some growth spurts can be traced back a generation so the height increase and weight gains can be in the same ratio of the grandparents.
Going back to your original question: "how much sleep to they need?"; that is a wide open question and is very individual. Like adults, some need less sleep than others, but there does need to be a routine.
The accepted levels are:
Age six : 12 hours
Age ten: 10 Hours
Age Fourteen: between 8 -9 hours.
However the above figures are not absoloute and depend strongly on the metabolism and fitness of the child, those who are fit tend to sleep less but deeper; those who are above the normal rates (and that does not mean they are obese or fat- and remember muscle weighs more than fat), it may be their metabolism is slower. As for a twelve year old, that can be between the two figures stated above in that age bracket. There is no absolute rule.
When a girl first starts her menstrual cycle it will be tiring, particularly with the loss of blood, as that wil reduce the amount of oxygenated blood in the citculation. Whilst most teenage girls' periods are quite light, and lose only a little blood, it does reduce the amount of oxygen in the blood stream as well as iron, the latter important for strength and the ability for blood to take up oxygen from the lungs.
If a child has physically worked hard, such as sport or digging, playing hard, e.g. climbing a tree, they will burn more energy and therefore need more calories to maintain weight; and to replace that energy spent and to allow the muscles to rebuild, will sleep longer and deeper. Also remember that sports makes the heart work harder, and is a muscle in its own right that is the reason why exercise is so important for both girls and boys.
When a girl becomes a woman and starts reproducing, the strong heart is important, since labour puts a lot of pressure on the heart, and therefore the stonger it is the better it will be able to put up with the strain if the heart has been "exercised" a lot when she was younger.
To a certain extent the same is true of boys when they reach an age when the first experience intercourse as the thrusting and energy required to ejaculate is quite significant in terms of energy spent.
Teenagers do need a regular sleep pattern, i.e. going to bed at a fixed time, and waking in the morning. However there is no reason why they can't have a "lie in" on Saturday or Sunday, so that the muscles etc can be ready for Monday morning. It is often a good idea for children to go to bed half an hour earlier on Sunday night to make up for the sleep, they may have lost during the weekend. It also helps balance out the energy spent on the sport or other energetic things like dancing etc.
ie... coming off of PS3, taking dinner plates to kitchen, leaving rubbish laying about, not getting changed for bed - sleeping in clothes, and not doing homework till very late night before - when ive asked him all week if he has any to do. 
