From this account there are 21 known death over 2 years in a country with more than 4 times the population of the US. So in a single rampage in a school guns made the equivalent damage of 8 years worth of knife attacks in school in China.
So China (a nation that brutally represses protests) has about 0.016 school massacre deaths per million people. So if we use your mathematics, if America bans and forcefully removes all of its 310 million nonmilitary firearms in its country it will have about 5 school rampage deaths per year due to knives, clubs, cleavers, hammers, based on the rate in China.
I think a more interesting statistic is how many gun deaths per million nonmilitary firearms in a country, but this statistic is hard to quantify as many nations ban guns, and the guns available in places such as China are hand made or stolen from the military or police.
Massacres are not a phenomena that only occur where people have access to firearms. The people that do these attacks are the anomalies of society. Having a mental health problem is not a predictable sign that an attack like this will happen. Nothing is going to stop a person that has made up their mind to do this kind of attack. The only possibility may be in reducing the number of victims per attack by restricting the ability of the rest of the population to defend their life. In exchange for that reduction we are talking about the possibility of the state increasing the suppression of its own population.