Because this survey, as described in you question, is done in only her class, it is relatively a small scale so she must include all of her classmates, otherwise she may get wrong results if she just take small parts into her survey
Since she is only wanting to know information on the people in her class, the population size is small enough that she can survey the entire population. This will also give her more accurate results than a sample would, since she would be asking every person.