Now, there are plenty of example of me answering questions with essays on why following validation rules are important. I’ve also said that sometimes you just have to be a rebel and break the rules, and document the reasons.
You can see in this example that framesets do work in HTML5 still. I had to download the code and add an HTML5 doctype at the top, however. But the frameset element was still recognized, and the desired result was achieved.
Therefore, knowing that using framesets is completely absurd, and knowing that you have to use this as dictated by your professor/teacher, you could just deal with the single validation error in the W3C validator and use both the HTML5 video element as well as the deprecated frameset element.