jaynerox,
There's a little truth in what your parents say. Full-fat milk, which is normally 4% butterfat content, is higher in calories than 2% or skim. If you are "too skinny" (how much do you weigh, how tall are you, and how old?), then this would be one way of putting more calories into you.
However, it's not altogether the best way.
Now, what I'm about to tell you should not frighten you. This process occurs slowly. It just starts very early in life.
You see, we have found that heart disease starts in the very young. The PDAY study (Pathological Determinants of Atherosclerosis in Youth) has found that heart disease starts in childhood. Those youth with the highest LDL, lowest HDL, and highest incidence of smoking, were the worst diseased, but every body has it.
Subsequent to the early findings, we have discovered that heart disease starts very early, as young as eight and in some cases, five years old.
American Army doctors in Viet Nam foudn that healthy-looking young men of 18 had fatty deposits in their arteries. This surprised the doctors and started much of our investigation into the causes of heart disease.
Full-fat milk speeds this process, for it adds fats, cholesterol, and arachadonic acid (a fatty acid that makes internal inflammation worse. Inflammation starts the process of heart disease, and the fats deposit in our arteries, plugging them and turning into the hard plaques that eventually cause heart attacks) to our diet and thus to our bodies.
So, what your parents are doing is not helping your long-term health. They are doing it with the best intentions, but it's not really going to help you.
Now, this doesn't mean that what they have done will kill you before your time. If you decide to drink skim milk (you'll have to get used to the taste. It is thin, almost watery at first, but you'll soon get used to it. I did, and now I wouldn't drink any other kind) and to cut back on most fat in your diet, the fatty desposits will slowly disappear. You can do this yourself easily. It may irritate your parents for a while, because it will be a change, and we don't like change, but if you keep at it every body will get used to it.
Just one warning: though the logic of this would tell you to become a vegetarian (I am one for this reason. It's a much healtheir diet) do not become a vegetarian yet. You are too young, and your brain and nerves need the fats that a vegetarian diet just doesn't have. If you should become one, do so after you turn 25. Until then you need to develop, and there are things we get from meat that we just cannot get anywhere else.
There are other ways to increase the number of calories in your diet if you are too skinny, such as high-protein, high-calorie milkshakes with skim milk and skim milk powder added. Smoothies made of skim milk with egg or whey protein powder added. Peanut butter and jam sandwiches. Fat-free sour cream on your potatoes. Pancakes with lots of maple syrup. Cereal with fat-free yogurt on top (along with the milk) and maybe even a line of honey on it. Chocolate cake (ok, ok, so it's my one weakness. Big deal)
There are many ways to boost your caloric intake. I just wouldn't do it with fatty foods like whole milk or cheese. They cause too much harm in the long run.