Why was the police response to the Columbine shooting criticized? Was the IMS used? If so, what amount of success did it have? If not, why not? What things do you think the police did wrong or handled incorrectly? What should be done in situations like Columbine in the future?

In 1999, when Columbine occurred, school shootings were relatively rare. According to Slate magazine (Cullen 2009), there were 37 school shootings in America between 1974 and 2000. While this may seem like a lot (over one per year), it is a very small number compared to the number of shootings since Columbine. As a result, law enforcement agencies were not well-prepared for such an incident. The law enforcement agencies in Littleton were ultimately criticized for being too slow to respond to Columbine, causing unnecessary loss of life.
There were two major failures in the response to Columbine. The first was that response teams were more trained to handle hostage crises than active shooters. As a result, their standard operating procedure was to surround the building, create perimeters, and then take steps to free the hostages inside. Less-trained forces, such as city police, were also typically instructed to wait for better-equipped, more highly-resourced, and highly-trained agencies to come on the scene rather than acting independently. Eric Harris and Dylan Klebold did not hold hostages; rather, they gunned down people inside the building. As a result, typical hostage-management techniques failed, and far too much time was taken up by using these traditional methods. 
The second major failure at Columbine was the lack of a systematic, well-run incident management system (IMS). While Mell and Sztajnkrycer (2004) noted there was an Incident Command System (ICS) in place and praised the successes of said system, they also noted several flaws. First, there had been few interagency training opportunities, which meant most of the individual responders had not been trained to work with other agencies. This led to issues; for example, one team not understanding another team's language, or confusion as to who had higher authority. Technology also became a problem, as the different agencies had different methods of communicating and used different radio frequencies. Finally, the lack of smooth interagency communication led to many mistakes, including an overrun of surplus volunteers that created many logistical and media challenges.
Unfortunately, there have been many school shootings since Columbine, and law enforcement has much more training now. If a shooting were to happen today, two important steps would be
1) Whoever is first on the scene takes control of the situation, including assessing if there is an active shooter.
2) If there is an active shooter, officers now follow the "active shooter protocol," in which teams of four officers enter the building as soon as possible in a wedge formation. The primary goal of these officers is to take down the shooter as quickly as possible. 
Additional improvements in technology and joint/ interagency training have also allowed law enforcement and emergency response to make great gains in containing shooting situations. A well-run, well-trained Incident Command System/ Incident Management System, with a clear chain of command, is critical in these situations.
http://ispub.com/IJRDM/5/1/12573

https://slate.com/news-and-politics/2009/04/the-four-most-useful-lessons-of-columbine.html

https://www.nytimes.com/1999/04/28/us/terror-littleton-police-response-school-attack-may-bring-changes-police-tactics.html