1/3 is like dividing the class into 3 groups with the same number of students in each group. 1/3 would be the number of students in each group.
Example: If your class has 12 students, then 1/3 of the class is 1/3 * 12 = 4 students because the whole class of 12 students split into 3 equal groups would have 4 students in each group.
1/3 also kind of means "divide by 3.'
Example: If your class has 18 students and 1/3 of them get lunch from the cafeteria, you can do 18 / 3 = 6 = how many students getting lunch from the cafeteria.