Is she in first grade? Is this her first year in school? K is so different in terms of being able to free play more etc. First grade really is a jump up in structure and following directions. Some kids with more subtle disability issues are not noticed to have problems until they run into harder and harder/more demanding environments. I would tell the school that you are concerned and that you are not OK with treating her as a "behavior problem" until you really check to see if there is something going on underneath all of this. A new baby can complicate things but generally would not affect all areas for a long long time.
Does she have insurance? I would ask her doctor. or call insurance and ask for how to get a referral to a Neuropsychologist. These are psychologists who have extra training in how the brain works in relation to behaviors. They look across many areas of development unlike a psychiatrist or neurologist (all of whom are good but just very different). Ask for a complete developmental evaluation to help sort out if there are any underlying problems. How long until she is 7? I would also get a complete Occupational Therapist (OT) evaluation (subtle motor and also sensory integration problems can really cause huge problems in behavior because the can't fix it on their own) as well as a speech/language path. evaluation for social communication, auditory and language processing, and general language understanding and expression issues. You might need to have a specialized audiology evaluation (the hearing testers) to look at auditory processing but start the other things first and by the time she is 7 then you will have more info to guide you in this area.
At school, I agree, Isolating her permanently is not a good idea. She may fully not be able to control her behavior or have a reason for the behavior choices that isolating her will only compound. She likely has skill areas she has not developed related to all the areas you want assessed. Ask the teacher for more information in a private conference. Get specifics. Find out how she is doing overall in school. Are there problems in teh halls, play ground, lunch, rest room, as well as class time??? How are academics coming along.
Gather your info then write (very soon) a letter and send it certified mail/return receipt asking for a full developmental evaluation in the school (this is in ADDITION to what you are doing privately. Include cognitive, academic, motor, speech, and BEHAVIOR (including an FBA=functional behavior analysis which helps them to develop a POSITIVE behavior plan to teach skills to help her behave better. These things are all mandated by law. They may tell you that they wont do it because she is not far behind. But if she is being isolated daily, she is socially behind. They are wrong if they tell you she has to be academically behind esp this early on because it is pretty hard to be really behind when this young... but it is important to start to look (and this is a process that may take a long time for you... to really find all the issues, but for them they must do the evaluation in a federally mandated amount of time...usually less than two months....so you can start helping her.) Early intervention is very very important so don't listen if they say they don't want to "label" her. they already have labeled her if they are putting her in a corner... with an informal and not helpful label. Bossy is not a helpful label nor is behavior problem.
Well, these are some steps to start to try to "figure it out" and believe me most of us have been there. It is not fun, makes us angry with our kids even when it is not their choice to have these issues, but we are human, right? We also learn to become strong advocates (here they say warrior moms, dads etc.).
Sorry this will seem overwhelming. I am not re-reading my response so please feel free to ask me or others if I have written something that is not clear, I am just rushing thru a little.
Nice to meet you and welcome to the board. Hope we get to know you more.