1. **State the problem:** We need to interpret the joint relative frequency $\frac{28}{612}$ from the given data about sports participation by grade level. 2. **Understand joint relative frequency:** Joint relative frequency is the ratio of the number of individuals in a specific category combination to the total number of individuals surveyed. 3. **Identify the categories:** Here, $28$ is the number of 12th graders who play two or more sports. 4. **Total number of students:** The total number of students surveyed is $612$. 5. **Interpretation:** Therefore, the joint relative frequency $\frac{28}{612}$ represents the ratio of the student body that are in 12th grade and play two or more sports. **Final answer:** The joint relative frequency $\frac{28}{612}$ represents the ratio of the student body that are in 12th grade and play two or more sports.