First, it is not part of the definition of ?theory? that it ?is something that has not been proven.? The National Academy of Sciences defines a scientific theory as ?a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences and tested hypotheses.? Even when ?proven? beyond any reasonable doubt, a theory remains a theory, and that does not imply that it is not also a fact. The real question is just whether or not evolutionary theory is well-substantiated.
The answer is that it is one of the most thoroughly substantiated theories in all of science, robustly meeting all of the above criteria. So it is not ?just a theory?: it is as well-supported a theory as we have. Here it is helpful to make a distinction between the claim of common descent (as an account of the origins of species) and the claim that evolution is to explained through the mechanisms of natural selection. Both are well established, but it is important to distinguish them in the context of the current debate.
The claim of common descent is a fact established by an overwhelming body of empirical evidence, and it is conceded even by most intelligent design theorists (as opposed to creationists). The claim that natural selection plays a role in explaining evolution is also typically conceded by intelligent design theorists. What they deny is just that these natural processes are sufficient to explain all the phenomena of evolution. Thus, they supplement existing theories with divine intervention somewhere in the process.
This leads to the second main point, which is that intelligent design is not an alternative scientific theory at all, but a theological theory meant to supplement science. Thus, while evolution and intelligent design may both be called ?theories,? they are not competing scientific theories with equivalent forms and degrees of support. Evolution is a well-supported scientific theory, while intelligent design is a theological theory with no support from the scientific realm except for the existence of currently unsolved puzzles, from which it is fallaciously inferred that science ?cannot possibly? explain certain things.
Such claims have been made often in the past, after which science progressed and explained the very things that had earlier been deemed impossible to explain scientifically. The history of such moves is therefore not promising. More importantly, while intelligent design could of course turn out to be true, it is obviously not a scientific response to current puzzles in science to jump to supernatural explanations for them.
The scientific response would be to expose the puzzles and then pursue scientific research projects to develop better scientific models to explain them. That is why intelligent design is not on a par with evolution, and has no place in the science classroom, though it has a place in broader public dialogue.