Ok so you are all cadets in the same military academy, you all study the same basics and are similar levels of intelligence and physical skill. That doesn't however mean you don't have your own areas of expertise. You are all 19 or 20.
These areas are summarized below and DO affect your stats, a full base stat sheet will be posted later and at that point I'll show what each specialty effects however it's fairly obvious if you think about it.
All marines can handle the standard handheld weapons with slightly varying degrees of ability, all are basically trained in driving the warthog and all have basic tech knowledge.
One thing that you must note (for both roleplaying and for a few background rolls I will be making if the need arises) is that none of you have ever seen actual combat before or even dead bodies. Unless you have a very good reason in your back story. Most of you are sent here by parents who are already in the military, high standing in society or wealthy while others sent you here to gain prestige and move up in society. Not all Grunts get special academy training, most get boot camps. You are expected to be the ones working your way up to positions of command.
A possible reason for you having seen bodies or combat is if you were present in an insurgent attack, you would have been a child or young teenager at the time and NOT a combatant. Include this with some detail in your backstory if you want but not everyone can be an 'insurgent orphan'.