underachiever

[ uhn-der-uh-chee-ver ]
/ ˌʌn dər əˈtʃi vər /

noun

a student who performs less well in school than would be expected on the basis of abilities indicated by intelligence and aptitude tests, etc.
a person or thing that performs below expectations.

Origin of underachiever

1945–50; under- + achiever
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019