This fall, I will be starting my freshman year of college, so I applied to an internship in the city of the college. It was clear in the interview that my availability over the summer, during the school year, and during the spring and winter breaks was a very important factor. I think my ability to work full time over the summer was also very important. They'd want to keep me aboard during the school year, even if working only 20 or so hours a week, but I would have to be familiar with the systems and procedures at that point to be productive. The time I would spend working there over the summer would give me enough experience to be able to that.
Due to my being nervous and not wanting to outright disqualify myself in the interview, I neglected to mention that I was invited to (and had accepted the invitation before applying for the job) a summer program run by the school which runs through most of August and will apparently require all of my time on the weekdays.
I've been given word that the wheel has started rolling on my becoming a part of the company, so I need to decide how to tell the employer of the program I will be attending. Given the understanding they already have of my limited availability, I'm not sure how close of a call it was to decide I would be worth the hire. I feel like bringing this up now might, for an understandable reason, prevent me from joining the company. It would seem extremely unprofessional, however, for me to wait to tell them.
How can I inform my potential employers about the summer program without ruining my chances of being hired?