Part 1

"How long have school shootings been an issue in the US?"

My rule-breaking, tongue-in-cheek, tl;dr answer is, "For as long as shools and guns have been around at the same time, school shootings have been an issue in the US." ;)

In all seriousness, this is a great question, and there are so many layers to unpack to answer with detail. It's also timely - my former home-state of Georgia is currently dealing with a tragic shooting that, at 8 deaths, exceeds the federal criteria for a mass shooting (4 deaths). Though that incident was not school-related, I think the question you asked here really speaks to a much larger issue about how we view incidents of mass violence in the United States. As such, I'd like to take some creative liberty to expand your question to, "How long has school violence been an issue in the US?" and I will, of course, speak to your point about school shootings more directly.

I'll divide my comments into 4 parts, which I'll briefly summarize here, in the event that you'd like to skip around.

The Linguistic Issue of "School Shootings:" I'll give some context to the words "violence," "school violence," "mass violence," and "school shootings," along with some explanation of why your question is way more difficult to answer than you probably thought.
The History of School Shootings: I'll give a brief overview of the first school shootings in history, and how they are conceptualized compared to the "Columbine-era" school shootings.
The "Columbine" Image of School Shootings: I'll speak directly to your question about Columbine. You "only" asked about an uptick since Columbine, but I think your question comes from a place of much more depth - hence all the exposition leading to this part of my answer.
Summary: Your Actual Questions: I have a tendency towards long, winding answers... but in this part, I'll just answer your questions as succinctly as possible.

The Linguistic Issue of "School Shootings"

Often, when folks ask questions about "school shootings," they have a "Columbine" image in their head: one or two offenders go on a "rampage," kill a dozen or more people, and then that blows up in the media. I find this to be a frustrating misconception for a few reasons. First and foremost, "mass" school shootings are simply not common, and neither are incidents of "mass" violence in general. In fact, violence, as a whole, has been steadily decreasing since the 1990s. Crime in general but especially violent crime, is practically at an all-time low. I have written in a previous comment about how we measure crime and the declining crime rate, but for a quick reference you can hop over to the FBI's crime data explorer tool to see that all arrest rates have been decreasing since the 1990s (that tool goes back to 1985). Arrests are, of course, not a perfect measure of crime (but not all crime is reported to the police), but homicide almost always does get reported to/documented by the police - so if you look at homicide rates using the FBI's tool linked above, you can very easily see the steady decrease.

Second, I have the same problem with media portrayal of school shootings that I do with the media portrayal of serial killers. This is not just news media portrayal, but all media, including shows and films. Namely, there are not enough serial killers to justify the sheer volume of serial killer media. <sup>Seriously, why can't I find one fucking podcast that isn't about "true crime"?????</sup> Similarly, there are literally not enough school shootings to justify the massively disproportionate media representation of school shootings. Let me put it this way: I like to think of "school shooting" as a term that is at the bottom of an umbrella-list of other terms used to explain multiple incidents violence. We have:

Violence:

A definition that varies from person-to-person, but when the World Health Organization declared it as a public health concern in 1996, they defined it as "The intentional use of physical force or power, threatened or actual, against oneself, another person, or against a group or community, that either results in or has a high likelihood of resulting in injury, death, psychological harm, maldevelopment or deprivation."

Easy enough, right? Well, how severe do these incidents have to be? Does "psychological harm" have to result in PTSD, or will a subjective term like "trauma" suffice to account for the word "psychological harm?" If the latter is the case, then I think there's a much stronger point to be made about the effect of the effects of school shootings, rather than the direct effect of school shootings themselves. In other words, there is a documented effect of secondary trauma stress on people who participate in lockdown drills, not even "real" shootings -- and we do lockdown drills far more often than actual school shootings take place (Stevens et al., 2019, paywalled). People in the community who aren't even directly impacted - like people in the general area during 9/11, or people in the general area during the Boston Marathon bombings - have documented traumatic responses (Garfin et al., 2015, paywalled)... far more people suffer from that sort of secondary trauma stress than individual incidences of physical, injurious violence combined, because for everyone 1 homicide incident, 1.5 to 2.5 additional people require mental healthcare (Miller, 1996) - excluding the psychological harm that doesn't directly result in mental healthcare.

Mass Violence:

Another term whose usage varies from person-to-person. According to the Office for Victims of Crime, the word includes things like domestic terrorism, war crimes, and other similar issues - though an explicit number is not given.

This word is subject to the same linguistic concerns with "violence," except now, you're tacking on another meaningless term - "mass." How many is "mass?" Maybe if we narrow it down to "mass shooting" it'll be more clear what you mean, right?

Mass Shooting:

Wrong. The Congressional Research Service and the FBI have a clear definition for this one: 4 or more deaths to a firearm. As the linked page notes, this comes with a caveat: "For example, if 10 people are shot but only 2 dies, the incident is not a mass shooting. Homicides by other means also are not counted. If 5 people are purposely run down and killed by an individual driving motor vehicle, the deaths do not count because a firearm is not involved."

School Violence:

I recognize that I haven't even gotten to the words that you used in your post yet, so let's narrow it down further and talk about school violence. We take the definition of violence, and apply it to schools. Hard enough, given all the problems I've outlined above. However, when you say "school," you further complicate the issue. Do you mean: School grounds? Indoors? Outdoors? What if the shooter was off the school grounds, but the victims were on the school grounds? What if it happened on a school bus? What if no one dies?

Violent interpersonal crime that results in death or serious injury is at an all-time low, as I already showed you. School shootings, as I will show you in the sections to follow, have historically been and continue to be, especially rare. What we should be more concerned about is the simultaneous increase in reported rates of bullying, larger classroom sizes, and fewer school counselors to accommodate (see, for instance, a 2005 report from the US Department of Education).

School Shooting:

Take all of the problems with the words for "violence," "mass violence," "mass shooting," and "school violence," and then compound them and ask a question about "school shootings." Let's talk about Michael Brandon Hill, who fired 500 rounds of ammunition at an elementary school, but no one died. This isn't a "mass shooting" because no one died. Some would argue it's not even "mass violence" because people were not directly injured in mass... and actually, the shooter wasn't even aiming for any of the kids; according to that article, he was severely mentally ill and aiming for "suicide by cop;" after all, he fired 500 rounds and injured no one.

So as you can see, there is a serious linguistic issue with your question. How many people have to die to meet the criteria for the question you're asking? My answer will differ dramatically depending on the language you use. However, I'll try to answer the "spirit" of your question without getting (too) bogged down in pedantry.