Creationism is not a theory in a scientific sense. Creationism is a philosophy with no empirical evidence to back it up.
A theory in science is a "testable model of the manner of interaction of a set of natural phenomena, capable of predicting future occurrences or observations of the same kind, and capable of being tested through experiment"
This means the Theory of Evolution is backed up by things that can be seen, tested, and predicted.
Whereas, Creationism relies on faith and belief, which cannot be seen or experimented on. And therefore, has no facts to back it up.
It would be the equivalent of studying Unicorns, Dragons, or Vampires in Science class.