Step 1: Understand the question.
We must explain what the term dropout rate means. This is a basic education term used to measure how many students stay in school.
Step 2: Break down the word.
To drop out means to leave something before finishing it. So a dropout is a student who leaves school before completing the course.
Step 3: Compare with the options.
Failing an exam is about marks, not leaving. Changing schools means the student is still studying somewhere. Repeating a class means the student stays in school. None of these is dropping out.
Step 4: Find the matching idea.
The option that says students leaving school before completion matches the real meaning of dropout.
Step 5: Understand why it matters.
A high dropout rate is a warning sign. It shows that children are not finishing their education, which planners try to reduce.
Step 6: Match with the options.
The fourth option is correct.
\[ \boxed{\text{Students leaving school before completion.}} \]