It is not always possible to perfectly gauge intelligence in school as there are most certainly multiple intelligence types: Visual/spatial, verbal/linguistic, logical/mathematical, interpersonal intelligence, etc.
One person can be bad at math and science, but understand history/politics and have an aptitude for language. This person can function like a human dictionary, and memorize entire historical events down to the letter and can learn to speak 8 or 9 languages. Another person can see numbers and visual patterns clearly but may not be able to as easily converse and have an intellectual debate using pure verbal concepts.
One person can figure out equations to make a craft that can fly at a faster rate due to nuclear propulsion. But he cannot rally support for his ideas because he does not have the interpersonal intelligence to manage and allocate human resources among large groups of people.
Etc, etc, etc...
To sum up my previous statements, people have different intelligence types suited for different things and it becomes hard to predict intelligence using general IQ tests.
When it comes to the military, I know a former Army Infantryman who served in Iraq and is now a portfolio manager at a bank in California. He makes at least $80,000 a year. After he finished his military service, he went on to university studies and got straight A's, earning an Economics degree.
I have a friend in my former Army Reserve unit (a Staff Sergeant) who has a Master's Degree from Vanderbilt University. At the university I attended, I had an astronomy professor who served 20 years in the Air Force.
People who finish their term in the military tend to either go on to trade school, or get their Associate's Degree, or get a Bachelor's Degree(or higher), and then tend to go on to do similar jobs as their civilian counterparts. I see no disparity in intelligence between the military and civilian world, as some posters are implying.