Use one of the following methods:
1) Instead of typing the common Ascii hyphen (-), enter the real MINUS SIGN U+2212 (−), e.g. by typing 2212 Alt X. The result is a more correct symbol, which is more clearly recognized as a sign, and it additionally disallows line break after it. The common Ascii hyphen is generally treated as allowing a line break after it, even when no space intervenes.
2) Instead of typing the superscript as normal characters and then using the superscript command in Word, use superscript characters: U+2078 SUPERSCRIPT MINUS followed by U+00B2 SUPERSCRIPT TWO. You can enter them e.g. by using the Alt X method (207B Alt X B2 Alt X). This is better than method 1, since you are using superscript symbols designed by a typographer, not reduced-size raised versions of normal characters, which is what the Word superscript command really does.
3) Enter the equation mode and use the exponent notation there. When you enter -2 (with common Ascii hyphen) as the exponent, Word automagically implements it properly. A minor problem with this is that you need to change the base “m” from italic to normal, as the equation mode by default italicizes letters. A major problem is that the mode uses the Cambria Math font, which pretty much means that you need to use Cambria (or Cambria Math) as the copy text font, to avoid typographic mismatch. But if you have more complicated expressions, that’s normally the best way to go (in Word 2007 or newer at least).