Brontosaurus never existed
Same thing, new accepted name.
TaughtBrontosaurus was a mistake. The name was withdrawn in 1903 and the animal is really an Apatosaurus. Stop saying Brontosaurus.
NowA 2015 study using a large statistical analysis of diplodocid anatomy concluded that Brontosaurus is distinct enough from Apatosaurus to be its own genus. The name is back, though not every paleontologist agrees.
What actually happened
In 1903 Elmer Riggs decided the specimen named Brontosaurus was the same genus as one already named Apatosaurus, and by the rules of naming, the older name wins. Generations of children were then corrected by teachers and museum labels for using the better name.
In 2015 Emanuel Tschopp and colleagues published a five-year quantitative study comparing 477 anatomical features across 81 diplodocid specimens, and found the differences between Apatosaurus and Brontosaurus were comparable to those separating other accepted genera. Brontosaurus was resurrected. This one comes with an honest caveat that most headlines skipped: taxonomy at this level is a judgment call about where to draw lines, and the reinstatement is accepted by many but not all specialists. What is certain is that the confident correction you received was less settled than it sounded.
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Who was taught this
Still standard through 2015, so anyone who finished school between 1950 and 2015 learned the earlier version.