According to Amazon.com, on this page: http://www.amazon.com/Boys-Start-War-Girl-Battle/dp/0440418410, this paperback version of the book has 144 pages. Other editions have other amounts... for instance, on the same site, the hardcover version is listed at 133 pages. Different paperbacks could have different sizes as well, and so then the number of pages could get bigger or smaller... the same would be true if they used different fonts or different font sizes.
'The Boys Start the War' by Phyllis Reynolds Naylor has 128 pages.
There is 104 pages is a boy at war
The Forever War has 236 pages.
The Parafaith War has 480 pages.
The River War has 1000 pages.
The Uplift War has 506 pages.
Wisdom of War has 320 pages.
Wizards at War has 560 pages.
Highways to a War has 450 pages.
Metaphysics of War has 171 pages.
The Holy War has 191 pages.
The Illearth War has 527 pages.
The Never War has 352 pages.
War with the Newts has 240 pages.