Oddly, very little.
In Virginia (where the shooting occurred) many schools have instituted new warning systems and have incorporated text messaging into the system. If there is an alert you will get a text message.
Most schools now practice fire drills more often. Now, a fire drill involves the evacuation of a building so it can be used in case of a fire or in case something occurs that requires the evacuation of the building.
However, there have been drawbacks. Virginia Commonwealth University actually got a law passed making it illegal to possess a weapon on school grounds. This is problematic because VCU is an urban campus and you have a mix of private property in between the school buildings. So, lots of non VCU people are in the area and it's very spread out so you can go days without seeing a campus police officer. Also, VCU used to have one of the top criminal justice programs there and the law has made it difficult for many part time students who are currently in various aspects of the LE field. This can be suggested as a concept of focusing on the law abiding (which is easily regulated) as opposed to focusing on those active in breaking the law. Thus this is considered a "Mala Prohibita" argument (Bad because it is not allowed) versus a "Mala In Se" (Bad in itself) situation.
With the recent budget crisis, many schools have cut back on their manpower in their security/police departments. In any budget issue, employee pay and benefits is the biggest budget issue and the one most easily managed by laying off personnel.
Now a lot of the security is done just to make people feel safe but really does not offer very strong protection or it is security that is easily avoided. This is a situation of creating a "show" and having the appearance of providing safety which gets a larger response from the public. However, there was a paradigm shift in which law enforcement will now actively engage a shooter as opposed to setting up a perimeter and attempting to treat it as a hostage situation. This is both good and bad. This creates a police intervention of the situation as soon as they get on scene but is bad because the situation is confronted without proper intelligence on the issue and also without proper backup giving the advantage to the criminal.
I hope that this helps.