Sorry, I didn't know this was a thread on mass shooting prevention in the US, I thought it was a discussion on the thread title-- "gun control". Particularly the raging argument between civilized society and the NRA/gun lobbyists/2nd amendment nutjobs.
However, to address that specific incident, and the inevitably of others in the future, there is no way to mitigate future shootings via any law, plain and simple. The US has walked so far down this dark path, the country is already supersaturated with heavy weaponry. One could make a law that bans guns entirely, and it won't matter at this point as there is so much availability outside retail channels already.
IMHO, all you can do at this point sadly enough is in fact place armed guards in your schools. It's the price paid for an increasingly class divided society, coupled with costly mental health care, with easily accessible assault weapons..
Guard/secure your schools, enact tough gun laws, start a governmental gun buyback program, and then maybe 20-30 years from now the damage can be undone.