As far as armed security goes, going back to my training back in the day, some basic hardening of recruitment facilities could do quite a bit.
While sandbags would be 'cheap' and effective, they're rather obvious. However, there are films that, while not protective enough to prevent 5.56 going through it in the space needed to reach a driver, will still drastically deflect and deform any bullets passing through it. These can be applied more or less discretely, so a shooter that opens fire from outside will find his fire mostly ineffective, which gives the marines time enough to find better cover(heavy metal & wood desks, perhaps?), and assume defensive positions in case the shooter tries to come in.
A 9mm on their hips with 'discretely' placed rifles will give them all the firepower they need.
Oh, and an IDS isn't that hard or expensive - it's basically a burglar alarm.