punctuation - Spaces and em-dash

In using the em-dash, where do you put the spaces? Which of the following is correct?

School is based on the three R’s —reading, writing, and ’rithemtic.

or

School is based on the three R’s— reading, writing, and ’rithemtic.

or

School is based on the three R’s — reading, writing, and ’rithemtic.

Answer

If we can trust Grammar Girl's thoroughness of search, then her advice is sound:

Every style guide I checked, except the AP Stylebook, stated there
should be no spaces between an em-dash and the adjacent words. That
means it is a style choice. If you're writing for a newspaper,
magazine, or website that uses Associated Press style, put in the
spaces. Otherwise, leave them out.

So:

School is based on the three R’s—reading, writing, and ’rithemtic.

That said, em dashes are often substituted with en dashes in British English, and these are spaced when used parenthetically:

School is based on the three R’s – reading, writing, and ’rithemtic

(Also, some British English guides are more or less strict about the use of the serial comma – the above sentence may be seen as: "[...] reading, writing and ’rithemtic").