I seem to be having trouble fitting in with new co-workers and am not being included in important decisions?

I am a school counselor and I work in an office with one other counselor and a secretary. I began working in September and, after 3 months on the job, still feel like I am not part of the team. I ask lots of questions and am usually responed to with a roll of the eyes or a look of confusion. I am not an idiot--I have a MS and graduate cumma sum laude. The head counselor makes most of the decisions (which is fine) but she never tells me about them--she does tell the secretary though. I end up looking like a fool in front of colleagues. She even went so far as to change the testing schedule I was in charge of and did not even bother to tell me she did so, or why. The clencher is that my principal came to me Friday and told me my co-workers do not feel I am being helpful enough. I explained that I am always asking if they need help and am usually told no. I also mentioned that I never received a job description so I do not know what else I should be doing. Any suggestions?

You have identifed a serious situation. Like an iceburg, only 10% is above the surface. Meaning there is more going on than you see.
There are some steps you can take, but if you have not been in this situation, you may be out matched.
It is a classic strategey, Exclude and then Blame.
You may be smart, and that may be the problem. It sounds like the old cliche, but outsider's who have talent, skills and actually care about the job, in this case counseling the students, will be pushed out.
You may need some outside on-going advice. I will email you or you can contact me.
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You are experiencing what is called a bad fit. It's simple. You don't fit them, they don't fit you. It doesn't mean you are a bad person, just a bad fit.

Each workplace has a culture, which usually evolves somewhat haphazardly and without direction. It just evolves. You have been placed in an environment that doesn't gel with you.

Maybe your predecessor handled it differently, possibly poorly. You've now been put in a situation which may prevent your success.

Discuss this with the Principal, ferret out the root cause. Be prepared to move on, maybe not immediately, but soon enough.

Nasty situation. Control issues, power issues, 'she's-got-more-experience-than-do-and-I... show-her-who's-boss', performance anxiety issues lol. Unfortunately, these are not uncommon issues in a very traditional, structured workplace.

My partner is a teacher. My understanding of the process is that you're required to have a 'conversation' about this situation with your colleague first. Then, if there's no change, you speak to your union therreafter. Sounds like a good time to have some mediation assistance. It's bad now and will only worsen the longer it's left to fester.

Good luck