That's because the shooter was still alive and shooting when LE arrived on scene. LE encountered the shooter, a fire fight ensued, and the children were caught in the crossfire. Many of the children who died were likely shot by LE. Think about how many LE were seen at the scene carrying very high powered, long barrel weapons ?

Much has been made of broken windows supposedly caused by LE when attempting to gain entry to the school. Those officers would have wounds from climbing through broken glass - why were there no pictures of those officers afte the fact ? There were enough parents with cell phones, media, etc . The windows were broken because LE shot through the windows, striking several children in the process.

Perhaps it came down to sacrificing the lives of 20 for the lives of many, many more who were still in the school.