Is Windows Os because

It depends on the version you are commenting on. Any Windows OS before XP, I personally believe can be thought of as a GUI for DOS. But as time goes, Microsoft is doing more and more to phase out the DOS command prompt. Some purist, like myself, still use it from time to time out of pure comfort. We are use to it, comfortable with it, therefore we still use it.

So, to answer your question, both your teachers are right. It a question of perception, I think. I personally think that Windows stopped being a GUI for Dos at XP and on. Some can disagree.

Of course, Microsoft says that Windows is an OS, not a GUI.

TCB

Windows 1, 2, 3, 3.1, 3.11, 95, 98, and Me were all DOS based.

All other versions are not (Windows NT 3.5, NT 4.0, 2000, XP, Vista, 7, and all of the Windows Server and 64-bit versions).

window manager, that changed from NT onwards.
Even that position is iffy, in that an OS is a set of routines that lets you access the hardware in an abstract manner, so could could argue any desktop is merely another way of abstracting the operating system.

Hardly worth arguing about really..