While my kids' school is not that small, far from it, they are part of a small choice program that means that literally they will be in the same class with the same kids every year until 6th grade, with a few exceptions for maybe 1 or 2 kids leaving/coming in every year.
It's been interesting for me to observe the dynamics in their classrooms. it's very different from my own experience (I never lived in the same place for longer than 18 months until I was 16), where I met all new classmates every year.
I think cliques can happen no matter what. I think developmentally kids go through some social hard times in the 3-6th grade years for a variety of different reasons at each stage--but that happens with all new kids or all old kids to be honest. What IS really interesting about my kids' classes is that the kids almost seem to have sibling relationships with each other, especially by now in my daughter's class (most of the kids have been together for 3 years now). Sounds great, right? Except for what do siblings do? Not only do they love and protect each other (hopefully), they also know exactly how to (and occasionally relish) push each other's buttons. They get sick of each other. They squabble. They don't like that s/he is looking at them. They get annoyed at each other. Luckily all of our teachers know about this family dynamic and are for the most part really good at helping them through it. I have seen sibling-esque (minus violence) throwdowns/hissy fits go down. OTOH, in 2nd grade when two children in particular were being bullied by a non-program 6th grader, not only did he have 22 OTHER 2nd graders going for him (not physically again, but verbally as in "leave my friend alone! Go away! What are you doing!"), making sure everyone was safe at recess, but also all of THEIR blood sibs and their sibs friends. You might fight like family, but anyone else tries to do anything and they better watch their step.
Is everyone best friends? No. Do the third graders occasionally indulge in "You're not my friend!! (this half hour)"? Yes. Are there best friend/groups? Yes. Has the teacher had to talk to the class about bullying being about more than punching someone, that it can also be writing mean things or saying mean things or teasing someone after they ask you to stop? Yes. But is it an overall safe environment? Yes. They're learning those things together.
I think it largely depends on the administration, teachers, and the parents as to how that plays out. We have a pragmatic, no-nonsense administration that is very serious about making sure that the environment of school is physically, emotionally, and verbally safe. We have teachers that know the kids AND the families AND happen to be really really good at helping kids work through their conflicts (and because of all the parent support, they also have TIME and space to do that as needed). The parents are a family like community as well (with squabbles, s/he's looking at me, button pushing, but also ferocious support of one of our own when needed) though we tend to view ALL the children as *ours*, which helps support the safe space and helping them learn how to be safe and nurturing with each other.
I think you can have an unsafe chaotic small school, and a welcoming, nurturing, loving big school. i think that always has far less to do with the kids than the adults.
so what are the adults like at this small school?