The interviewer is asking if you're available to teach summer school to gauge your level of commitment to the job. It's also possible they're just trying to get a sense of your availability in general or to see if you're willing to go above and beyond what's required of you. Regardless of their reasons, being honest and open about your availability and willingness to take on extra responsibilities is important. If you are available to teach summer school, say so. If there is a reason you are unavailable over the summer, explain why. If you have a valid reason, they may not hold that against you in the hiring process.