I think you are misinterpreting the interviewer's motivations entirely, and understanding them is important to answering the question properly. There are two possible mindsets the interviewer might have:
- "I like this applicant. What a nice young person. It sure would be a shame if 20 hours of work a week messed up this person's school work. I should offer some advice, like I would give to my own child or my younger self, and suggest that working while in school isn't always the right thing to do."
- "I am worried this applicant will quit working for me and leave me in the lurch if the job starts to interfere with schoolwork. I hope that has all been thought about and there's a good plan for managing work and school at the same time."
The first motivation is not going to affect your chances of getting the job. The second might, so you need to answer in a way that shows you've thought about it, and you know you can manage both. You're confident that 20 hours a week leaves plenty of time for your studies, so you say that. You might also take this as a chance to express how much you like the industry you are studying to join, your excitement at getting to do real work on real projects, and your belief that what you learn about the industry on the job will actually raise your grades.
However you seem to imply in your question that you think the interviewer's mindset is more like this:
- "My job is to make sure nobody gets hired. I'll find some way to stump every applicant, knock them down, ask them something they can't defend against. Vulnerable applicants will never get past me!"
While the occasional interviewer like this may exist, interpreting all questions as though they come from this mindset will hurt you in all your interviews. When you're asked a question, don't think about defending against it. Think about what the interviewer is trying to find out. When they ask "how well do you know C++?" it's pretty easy to establish they are trying to find out how well you know C++. When they ask "you're still a student, you're so young, working could hurt your grades or make your degree take longer, are you sure you want a job now?" they're trying to find out "is it worth hiring you when you might just quit if school work gets hard?" or "do you really want to work here or do you just need money?"
Guessing what they want to know enables you to answer the question in a way that makes you more likely to be hired. Not by lying, trotting out a prepared glib answer from "how to win every interview and always be hired" but by telling the truth about what is important to you, what motivates you, and the effort you've already put into your career (in this case, thinking about how to balance school and work, and the benefits to your studies of some industrial experience.)