Here's how you can answer, depending on the context:
If Juan has boys in his class:
* Sí, Juan tiene muchachos en la escuela. (Yes, Juan has boys in school.)
* Sí, Juan tiene varios muchachos en su clase. (Yes, Juan has several boys in his class.)
If Juan doesn't have boys in his class:
* No, Juan no tiene muchachos en la escuela. (No, Juan doesn't have boys in school.)
* No, Juan no tiene muchachos en su clase. (No, Juan doesn't have boys in his class.)
If you need more information:
* ¿A qué te refieres? (What do you mean?)
* ¿En qué clase está Juan? (What class is Juan in?)
* ¿En qué escuela está Juan? (What school is Juan in?)
Remember, you can also answer in a more casual way, depending on your relationship with the person you're talking to.
Let me know if you have any other questions!