Learn English – Word or a phrase that means your results could be biased toward what you predicted them to be

I believe the word you're looking for is stereotype. Stereotypes may be treated as a representation of the "typical" group member, but in reality they are most often a characterization of the stand-out traits of "highly visible" members. These members might be known as extremists, depending on the type of group (e.g., religious extremists). They also might be known as geeks (e.g., science geeks).

If you really want a "single word", you could go for something like oft-ignored, oft-forgotten, oft-overlooked, but I seriously doubt there's anything (based on that construction or not) that would disambiguate between things students fail to revise for, as opposed to things the teachers fail to review. Or indeed, anything that significantly implies such things are always encountered after that lack of preparation.