I know that when an email has been compromised that I can go to my profile and click on my login and get rid of an email. I have an issue where my professor suspects me of plagiarizing off my own post from Codereview. In fact, several people from my class took my code and turned it in. I told the professor it is a program that I posted using my account. I have yet to see her in person but I told her I can prove it by logging in. The professor said that it is easy to change an account email if you know the person. I have only ever used two emails on SE, but I don't have the original confirmation emails besides an email that said I posted a question and need to verify my email and another one that says to complete my registration. The good thing is that they correspond to the day when my account became active. However, is there any other way to validate that this is my account and show the emails that have accessed it?
First, it's refreshing that you are having this problem instead of the far more common problem of professors considering using our sites as a form of cheating. (For more on that, please see how our Mathematics Educators site answered "How to assign homework when answers are freely available or attainable online?")
Second, you can prove you currently control the account by changing your "About me" section to include a message to your professor:
Hi, Professor $last_name_of_professor! This is $your_name_here, your student. I'm the one who wrote: $link_to_post_revision.
We occasionally use a variation on this technique to confirm ownership for account merges.
Finally, if you still need help, please use the contact form at the bottom of any page to ask for more details email address you've used to register your account. The scenario you describe of taking over someone else's account is actually difficult to accomplish and readily noticeable in our logs.