It's always interesting to see what bits of "common knowledge" change as the years go on. I once had a teacher say I was being silly for saying lightning could hit your water pipes while showering, and it turns out that's totally true. Here are just a few more things that we learned in school but turned out to be completely bogus.
Pluto is no longer considered a planet
It was pretty debatable from the get-go, but NASA officially confirmed it in 2009.
Both halves of an earthworm don't stay alive if you cut one in half
Turns out that's just the synapses in its body dying out. It's very much dead.
Dinosaurs had feathers
It probably didn't help that we were all obsessed with Jurassic Park, but dinosaurs, and especially Velociraptors, were more like walking primitive birds than giant lizards.
You don't have to wait 30 minutes after eating before going swimming
Our parents thought our bodies would legitimately seize up and we'd drown. Turns out you just have to try not to throw up.