Hi J.,
My suggestions: (and it's a bit long)
Talk to his teacher. Ask her questions...in a respectful, really wanting to know how she works with and guides her students. Understand that teachers have an incredibly difficult job today....even with kindergarders:), and some of the more experienced (older) teachers who have been doing this for a while can get set in their ways and just don't nurture our children in a way that we would like them to be nurtured when they are out of our care. Then there are the amazing teachers that we wish would follow our kids throughout their entire education:) The point is, we need to work with all sorts of people, philosophies, and personalities throughout our life...so understanding where they are coming from might help you figure out your next step.
Then, ask your son, "how is your teacher mean?" "what does she do when _____ happens?" Helping him open up more about exactly what is going on in class could also help you understand what his "mean" really is. You can also ask, "what one thing did she do today that was really cool or nice?"
See if you can talk to other parents if they are experiencing the same issue. If the teacher really isn't a good fit for your son, see if he can be changed to another class. Though we want to encourage our children to learn how to make the best of an imperfect situation and learn how to work with all sorts of people, it's also important that your son is having fun in school and on the right track to a lifetime love of learning...so if he cannot get the extra attention he might need right now, a different teacher could be an option. But before you make this decision, really try to work with his current teacher and also help your son transition into a new environment with a new teacher. It could be that he just loved his Preschool teacher so much, that any other teacher is just "mean" :) Also, he's not only having to transition into a big kid school, but also transitioning into being a big brother at the same time.
Another idea....what has your gut been telling you about our education system or his school? My kids are in a public school...and we are fortunate enough that it's based on a non-traditional philosophy, but I feel that our country's education system is really lacking in so many ways. Most kids get a good education, and most schools are great, and there are alot of incredible teachers out there....but these great schools and great teachers get lost and pressured into "performing" instead of really teaching the whole child. I have friends who are teachers and they get very frustrated that they can't really teach the way they want because of the system.
Plus I feel that sending 5 yr olds to school all day is assinine - despite all the research I read on getting an early start....5 is still too young to start school. In Japan children start school at 7 yrs old! Yes, they are more structured, less down time,less play and social time, more of a conformity culture....but children still start later and end up ahead academically of U.S. kids.
So where's the balance? I don't have an answer...it's not easy, but I beleive it needs to start with parents who are willing to question those in charge, work with those in charge, and even challenge the system until our children get the education they deserve. I know this seems like I'm on my soap box - and I guess I am a bit :)....but having 4 children - and yes 2 of them started at 5 (before I thought differently), one started school at 6, and maybe my youngest I'll homeschool until she's older. I know that you want to make the best possible decision for your child and I know the stress or worry that comes along with making those decisions...even the "is my child's teacher too mean, and what can I do about it?" You know your son the best. You are being a responsible mom.
Talk to his teacher, talk to your son, talk to the other parents, talk to the school counselor, research homeschooling, put your 5 month old in her sling or stroller and volunteer in his classroom, look at all the options, think in mindfulness and then go with your mamma gut intuition.
Much good energy to you and your family,
A.