Let's start by calculating how many students does 24% represent.
24% * 24 = 0.24 * 24 = 5.76
We got a decimal number as solution. The reason why exactly 24% students can't be in the play is because this percentage does not represent whole number of students. She can't take 0.76 students
If she wanted 25% students to be in a play it would mean she has to take:
25% * 24 = 0.25 *24 = 6 students