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As far as I know, DNA is the construction protocol of all organisms on Earth.
Does it change when influenced by time and environment (physical laws)?
As parents with schizophrenia are more likely to have children with schizophrenia and an individual with no genetic predisposition for schizophrenia can suffer from such a condition because of drug abuse or environmental influence, does this prove that human DNA is changing during our lifespan?
To quickly answer your question, yes DNA changes over the life time of many organisms including humans. You have a whole host of mechanisms in your body that try to prevent your DNA from changing, but they are not perfect.
A good example of this is DNA degradation due to aging. When discussing aging in biology, we use the technical term senescence. Of interest here is the shortening of telomeres due to mistakes made in your cells as they continue to divide (copying the DNA to each new daughter cell).
In a grown adult, there's not some master well of DNA somewhere such that if you change it there it will immediately change the DNA throughout your body. Most of the cells in your body have DNA in them (red blood cells are just one example of cells that don't have DNA in the traditional sense). Most of these cells can and will divide over time (mitosis) to produce new cells with copies of that DNA. However, there are also plenty of cells that don't divide often or at all in healthy adults. If one is concerned about schizophrenia, it would be important to note that many neurons in the CNS are in this group of static cells.
Considering environmental effects, there are plenty of things that can change your DNA. Radiation, gamma and x-rays in particular, do a great job of disrupting, degrading, and occasionally mutating DNA. The class of chemicals and physical agents that can change DNA are called mutagens. This shares a root with the verb mutate, which is when something is acting on genetic material to cause it change.
You will note that I said genetic material, and not DNA. Without getting too far into the debate of what is "alive," DNA is most certainly not the only "construction protocol" for earthly organisms. RNA is one common alternative, but there are weirder ones that are hard to describe like prions.
A DNA change will only be passed on if it occurs in a germ line cell (the cells that become eggs or sperm). If you have a mutation in, say, a liver cell, that will not be passed on.
But I dont think thats what you are talking about. From reading your question and comment closely I think the answer is "no". A person who becomes schizophrenic because of drug use does not become any more prone to having schizophrenic kids than a person who doesnt do drugs. Drugs can upset the chemical balance in your brain and that can mimic certain genetic diseases, but what happens in your brain because of drug use can absolutely under no conditions be passed on to your child.
In your piano comment things are slightly more complicated. Being good at playing piano is a result of a complex interaction of many factors, some of which may be genetic. A long line of pianists generation after generation may be a result of genes that, say, sharpen a persons auditory perception so they can hear and interpret notes better. However, I am sure that the parents encouraged piano playing and practice for their children, which is just as important.
But most importantly, playing the piano a lot will not in any way cause changes to your DNA that will make your children better at playing the piano. Your genes dont care how well you play the piano. Mutation is random, and that is the only way DNA can change.
It's become quite clear that your active genome changes all the time, due to various effects. There is also gathering evidence that epigenetic changes can occur in somatic cells and be inherited by the offspring. But these seem to be stress-related.
So, for schizophrenia: If the family had a gene that triggered schizophrenia, which was normally inactive, then stress/drugs could cause an epigenetic change which would make the "schizophrenia gene" active. And this epigenetic change could conceivably (see what I did there?) be passed along to offspring.
