When do I let my interviewer know?

Do not go to these sites. They will steal your credit card information.

Don;t put it off, better to not get hired than to get fired for not telling them you are not available to work the hours they need.

There is no “correct time” to tell the interviewer that you cannot fulfill the duties of the job for which you are applying. Find a different job.

Don't mention it at the interview, but if you get an offer, tell them then BEFORE you accept the job

Let them know immediately. If you don't value their time or resources, why should they value yours? If you take the job, knowing full well in advance that you may not be able to fulfill the requirements of the job, then you're no good to them as an employee.

I'm a union man raised in a union family. I'm all for worker rights and I tend to side with a worker initially on anything. But I'm also for worker responsibility. You have a responsibility to be honest with your potential employer, since they're spending money and resources on just talking to you. Those are resources that can go towards someone else's interview; someone else who'd be fitter for the job.

So, walk in, introduce yourself to the interviewer and get the pleasantries out of the way. Then tell them about the potential conflict. "I don't want to waste your time or anyone else's, so before we begin the interview, I need to tell you that the hours you're requesting for the job may interfere with my school schedule. My classes are from 6:00pm to 9:00pm, so I won't be able to work until 6:00pm every night."

It's that easy. They may turn you down, but they won't be bitter about you. You may be able to apply again in the future if a position opens up that won't conflict with your schooling.