Student protests are back in the news. The outcry against the treatment of students of color has spread from Yale and Missouri to Ithaca College and Claremont McKenna. On Facebook, students and young alumni around the country have asserted their solidarity with protestors. As Vox’s Libby Nelson puts it, it’s possible that a “long-overdue racial reckoning” on campuses has finally arrived—and that’s on top of the already-planned national protests against tuition hikes and rising debt.

So in the coming weeks and months, students nationwide may march, demonstrate, sit-in, and occupy quads and buildings. But they’re unlikely to rely on a tactic used elsewhere in the world: a general strike. Why is that?

Though there’s some dispute over the definition, a “student strike” is when the vast majority of a school boycotts class en masse on a certain day. Sometimes the faculty does not attend, either, effectively shutting down a university. Student strikes can happen on a national scale—hundreds of thousands of students skipping class—which, in turn, closes hundreds of campuses.

Student strikes are rarely the primary form of student activism, but they’re much more common in many places than they are in the United States. Some South African students, for instance, are currently striking to protest tuition hikes. Québécois students have been in and out of strike this year, though not to the levels seen in 2012, when about 250,000 young people participated in boycotts or demonstrations. And the past decade has seen large-scale strikes in Greece, Puerto Rico, and Chile.

Yet not in the States. “There has never been, in the last 40 years, a large scale, coordinated, national—or close to national—student strike,” says Angus Johnston, a historian of student activism and a professor at the City University of New York. He offered two broad reasons why that is the case.

First, Johnston says, many American students aren’t aware of striking as a protest maneuver. That would make it difficult for organizers to orchestrate the huge numbers of participants needed to make it work.

“You need to have an awareness of it as a tactic, because it’s something you can’t just do with a small number of students,” he told me. “If it’s a small number of students, then you just fail your classes, because you’re marked as absent. It needs to be a tactic that can be adopted on a really, really wide scale the first time out.”

The few times it’s been attempted here, that happened. The most prominent postwar student strike in the United States came in the spring of 1970, after National Guardsmen shot and killed four undergraduates at Kent State. Students and faculty around the country—mourning those deaths as well as a similar shooting at Jackson State, and protesting the recent American invasion of Cambodia—struck. They shut down almost 450 campuses and demonstrated on nearly 400 more. A presidential commission from the period estimated that a third of all American campuses were involved somehow.

But even that incident revealed how little American colleges were prepared for strikers. “The reason why, honestly, the shootings happened at Kent State is that the university and the state refuses to shut down the campus when students went on strike,” Johnston says.

But in nations where students strike today, he told me, universities have different protocols: “In Canada, you have a situation where if you get to a certain level of student involvement, you can be confident that in most cases at least the university will be shut down—because that’s the protocol that’s followed.”

This is itself a function of local history and custom. Anglophone students in Canada, especially beyond Quebec, are much less likely to strike than their Francophone counterparts. Some European nations have strong traditions of strikes: Classes in Croatia were shut down as recently as 2009, and planned student boycotts famously helped set off the French civil unrest of May 1968. Yet in Britain, Johnston added by email, “marches and rallies and occupations are far more frequent.” And even in places like Quebec, “student strikes are often quite controversial even among activists.”

But law and history are not the only factors pushing students to avoid striking. Americans don’t strike today, Johnston also believes, in part because they can’t afford to miss class.

“Students in the United States today are living in conditions of economic precarity that didn’t exist in the 1960s,” he said. “As students have gotten poorer on average, tuition has gone up. And so they’re getting squeezed on both sides. They have a lot less ability to withstand the effects of … losing a semester, because if that happens, they’re gonna be screwed.”

In 2013, the Census Bureau found that more than half of students who live off-campus live in poverty, a cohort of young people so large that it elevates the national poverty rate by almost a percentage point. And a more recent study found that the vast majority of families who receive Pell Grants have no savings.

Johnston said that even in his own experience as an undergraduate, he’d seen the economic consequences of boycotting class threaten to splinter student solidarity.

“I remember at CUNY in the early 90s, there was a real tension,” he told me, between the liberal arts students and those in more pre-professional programs. “For instance, my mom was teaching nursing at the time. A lot of her students were like, ‘We gotta stay in class. You’re saying we’re going to shut the university down for the day, but I’ve got qualifying exams coming up and I can’t afford to lose this day.’ I think those pressures have only intensified in the 25 years since.”

Yet maybe American students have already found a way out of this conundrum. The protest tactic that rendered the Missouri protests so suddenly, stunningly effective was a de facto strike—first by many of the university’s black athletes, then by its entire football team. Some critics of student strikes point out that, unlike workers’s strikes, boycotting lectures doesn’t collectively strangle a revenue-generating corporation: They just keep tuition-paying kids out of class. But college athletics departments don’t call football (as well as men’s basketball) a “revenue sport” for nothing, and striking cuts to its core. For his part, Johnston said he couldn’t think of any players’s strike similar to the one Mizzou pulled off. Perhaps college athletic strikes will become a peculiarly American form of an international institution.