A standardized score of -.63 means that the student's score is .63 standard deviations below the mean score of the class.
In a normal distribution, approximately 50% of the scores fall below the mean and 50% fall above the mean. Since the student's score is below the mean, we can say that the student scored lower than the average student in the class.
However, we cannot make any conclusions about how the student's score compares to the rest of the class without knowing more information about the distribution of scores. If the distribution is approximately normal, we can say that the student scored lower than approximately 73% of the class (since .63 standard deviations below the mean corresponds to approximately the 26th percentile of a normal distribution).
Therefore, the best statement we can make about the student's score in relation to the rest of the class is that the student scored lower than the average student in the class and lower than approximately 73% of the class if the distribution is approximately normal.