Es un bulo/ corre un bulo

Es un bulo/ corre un bulo
 

Cómo se diría en inglés es un bulo, un bulo es algo que todo el mundo da por asumido de que es cierto, y es completamente falso.

Mi diccionario da el sentido de rumor (rumour in English, rumor in EEUUense) o engaño (hoax), pero tu descripción parece más "old wives' tale" (cuento de abuelas) o "well-known 'fact'" (que es distinto de "well-known fact").

I think the best translation in English is rumour, but when we say bulo in Spanish we are speaking of a false rumour.

Pienso que "hoax" es una buena traducción porque un "hoax" siempre llega a ser falso, sin embargo gracias por la nueva palabra "bulo" :D

Then the sentence would be something like:
There is a hoax going on about teacher working in X, that they are all foreigners.

In that context "false rumour going round" (not "on": está circulando, no occuriendo, ¿no?) is best. I'm afraid that I'm struggling to define "hoax", but it certainly carries connotations of deceit.

Es un bulo ...

You would have to just explain it out, and say something like "It is something that everyone takes for granted, but that is wrong".

Silopanna

Well, a hoax is definitely about deceit but I think the distinction might be that it's a deliberate one, i.e. a conscious attempt to deceive someone often with a distinct purpose in mind.

However the way 'bulo' is described here in the examples perhaps 'rumour' or 'vicious rumour' would come closer to it not so much being intentional, just a falsehood that is going about of which people are not sure about its correctness or falseness? :)

Probably a bit outside the scope of what is intended here as this term is used more for science and not so much for normal everyday life I think, but there is the word 'fallacy' (i.e. false conclusions based on poor arguments and/or logical deduction and coherence) .