First find the standard error of the mean for a sample size of 31 =10/SQRT(31) = 1.796
How many standard errors are there between the observed sample mean and the population mean: (55-50)/1.796 = 2.783 standard errors.
The percent of area under the normal curve from negative infinity to 2.783 = 99.7%, meaning their class score is higher than 99.7% of all other samples of 31.