What a silly question! It's like back in the 70s, where some people believed in a “Chinese conspiracy” in which everyone in China planned to jump into the air all at once. Supposedly, their combined weights would cause the earth to tip over, knocking it clean out of its orbit. Why the Chinese would do this to themselves was never considered.
However, to address the question, “What would happen if all the world's volcanos erupted at the same time?”, I should probably take it seriously.
Jessica Ball, a geophysicist with the United States Geological Survey (USGS), envisions a supervolcanic oblivion. She points out that some volcanos are more dangerous than others (like it matters), but in any case, that the world's climate would change, perhaps permanently.
According to Paul Kimberly, a Global Volcanism Program Manager with the Smithsonian Institution, the five most active volcanos in the world are Sangay in Ecuador, Santa Maria in Guatemala, Stromboli in Italy, Mount Etna in Italy, and Mount Yasur in Vanautu. The largest active volcano in the world is Mauna Loa in Hawaii. Still, there must be hundreds of volcanos in the world.
Some volcanos are mostly dormant but could explode at any time. Other volcanos have continuous flows of lava slowly oozing out of them. Most volcanos are known for slower, calmer eruptions, the kind a person can easily outrun. Like Kilauea in Hawaii. Then there are taller, stratovolcanos like Mount Fuji, where the lava flows can barrel down the mountain slopes at supersonic speeds, splattering enough ash to blanket the sky in darkness for a very long time.
If all of the earth's volcanos exploded at once, we couldn't escape in a plane, because the plane has already melted. Then, the ash would rise and without sunlight, crops would fail and animals (like us, for instance) would starve. Then the world would be a cold, dark place. Anyone who breathes in any of that ash will die slowly of agonizing respiratory distress. People who stay inside buildings are in danger of structural collapse, because the ash on those rooftops is five times denser than water, and rooftops are not designed to hold that much weight. There would be no escape, unless people somehow knew about the eruptions many years in advance, and created some sort of underground city, with many tons of canned food stored away.
Eventually, huge quantities of methane gas would be released, causing global warming and smelling like a great big fart. Our planet would be a scorched, lifeless mess. Not that we are around to notice.
About three and a half billion years ago (how on earth do they know this?), Mars had such a prolonged volcanic eruption that it gauged out its own mantle, which then plopped unceremoniously onto the planetary surface. This event actually caused the whole planet to tip over 20 degrees, which (they say) is like if Paris suddenly moved to the North Pole.
If every volcano on earth erupted at the same time, the earth just might tip over.