Establishing truth is a tricky thing. In a court case, police and prosecuting lawyers will try to find absolute proof of someone's guilt or innocence, but this is often impossible. Eyewitnesses could be lying, the D.N.A. “wasn't mine, your honour, it was my evil identical twin's”, and the smoking gun tossed in the river may never be found. Real life is not as tidy as CSI-type television shows. Most often, juries are asked to make educated estimates of guilt or innocence based on a quantity of circumstantial evidence. So in practice, lots of evidence is casually equated with absolute proof.
Real-life science suffers similar problems. Newton's laws of motion are called “laws” because they can be absolutely proven—if you have available a frictionless vacuum chamber. However, proving them in an everyday school physics class is nigh impossible, although your results may come close enough to offer considerable evidence that your physics teacher is not just making these laws up. Technically speaking, the theory of evolution has never been absolutely proven, because nobody has yet been able to replicate the exact conditions of the big bang or the unicell-producing primordial soup in a laboratory. However, several experiments (such as the large hadron collider project) have come close to achieving this and considerable archeological and biological evidence must also be taken into account. Whether or not there is enough evidence gathered to be safely equated with proof, there is certainly enough circumstantial evidence that the theory ought to be considered in schools!
In order to deal with messy real-life science (and to prevent scientific fraud) international standards were established for how something is officially proven. If someone wishes to prove their theory by an experiment or survey, they must must meet reasonable standards of accuracy and non-bias. They must then publish their results for peer review. The theory is still not considered proven until another researcher in another location performs their own experiment and supplies independent proof for peer review. So it takes a lot of time, money, and manpower to officially prove even the simplest bit of science. If you are studying a multifaceted part of real life, the process becomes excruciating, since each facet must be independently explored, verified, and duplicated. If you are studying anything about people, there are also ethical issues and the complication of finding people willing to be experimented upon.
This all comes home to roost with