Charm School is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

As this enchantment enters, choose a color and balance this enchantment on your head. Prevent all damage that would be dealt to you by sources of the chosen color. When this enchantment falls off your head, sacrifice this enchantment.