Off topic: is it illegal for your boss to make copies of your network password and distribute it ?

Not sure about US law, but where I live, this would be perfectly legal.  The network access you are provided is in order to do your work and all your work, inluding emails is deemed the property of the company.  They are not breaching privacy by accessing any of it as you have agreed to that as part of your employment contract.

However, most companies would have an IT security policy in place that would state that passwords should not be shared etc, so while not illegal as such, it would be a breach of company policy and result in disciplinary action.

Do you know what your IT policy is?  

I would escalate it to seniors as you risk reputational impact, or even legal ramifications if someone used your login for some illegal purpose (ie. to send a threatening email, access illegal porn content etc under your name.)

 Thank you to all the posters....

Yes, we have a tech policy that all teachers are required to sign for employement.  In that it states explicitly that we are not to reveal our passwords.  I do not know if the administrators were required to sign this.  I concur that it opened me up to potentially somebody posing as me, but the intent of the administrator was not to smear me specifically.  What happened is we have Nwea testing in our building (a nationally normed standardized test that shows student growth) and he passed out all staff passwords to our new staff.  Now, this is the second time he has done this and the staff initially all felt it was so teachers could check out how bad the perceived bad teachers were... a teacher police type thing.  This administrator is all about bullying and encouraging staff members to eat each other if there is a perceived weakness.  Anyway, after the first time he did this I went on the Nwea site and changed my password over to my network password.  (My test scores are significantly higher than the rest of the staff and I was taking grief for it every so often so I just changed the password.)  Anyway, he passed it out again and that is how the network password made it's rounds.  Never connected the dots that he could see the password with his adminstrative access, not that it mattered he got it that way because it wasn't too long after that he required all staff members to give him their network passwords. 

The problem with taking it to the superintendent is that they are friends, and I highly doubt that my principal is going anywhere.  People may be trying to get him fired, but I think it is not likely he is going anywhere.  Hence, I am stuck working with him.  And if he freaked out this bad for me asking for a new password, I can't even imagine what he would do if it went to the superintendent.  

Seriously? FERPA! 
http://www2.ed.gov/policy/gen/reg/ferpa/index.html

 Thank you :)  

Why does he/she have your password to begin with?  In my school, no one has my password unless I gave it to them. 

 2 part answer- On a site that he has administrative access to he printed off all of the staff passwords and apssed them out.  These were not the network passwords, but just to access the results of standardized test.  The staff felt he passed them out so everybody knew there were eyes on each other and the test results.  After he pulled that stunt, I went in and changed my password and (without thinking) changed it to my network password.  2 years or so later he again passed out this list of passwords, which now showed my network password.  Aside from that, about 2 years ago he forced all of us to give him our passwords.  We have a new superintendent that just found out about it and the process is starting to quietly go to affected teachers and reset passwords.  They are worried if he gets wind of it that he will come down on me even harder- which is very likely.  

In my school, the principal has access to all of the passwords, but he shouldn't be distributing them.  Hopefully, your union will be able to intervene.  It depends on how strong it is in your state.  If you're tenured, go over his head.  Maybe make an appointment with the superintendent and explain why you're uncomfortable with having your password distributed.  It will create some tension with your principal, but it sounds like the tension is already there anyway.  Good luck!