My professor teaching us dos and bash, while I'm an ethical hacker.

@AtuM exactly :)

Dos like the old operating system or dos like denial of service?

@Lensflare nope... The old one...disk operating system :-)

You can be an ethical hacker and still not know that, so, sit in that lecture, or leave if you're so confident.

Even if you're at your first year of uni or Masters, chances are, there will be students who are newbies in the topic and who may not know half of what you know in that topic.

Having been in that situation in various stages of my education, I can tell you that it's up to you to find something to do while the others are "brought up to speed".