My twins weaned at 5 1/2 and 6. Although we do homeschool, we are enrolled in an independent study school with the school district, so they are in workshops and meet with a teacher. It was never an issue. I don't think anyone knew they were still nursing, because it just never came up. It was simply part of our routine, and by that point they only nursed once or twice a day, so it wasn't something that their peers would have witnessed. It's like brushing teeth before bed - have you ever heard little kids talking about that? Why would they if it's a normal part of their routine?
I have several friends who have continued to nurse into K and 1st grade, and they also report that it rarely comes up. When it has, it's been positive; their peers haven't absorbed society's attitude that breastfeeding is only for babies and are typically curious and open about it. I think it will help to talk to your child about differences in families, about how many children wean earlier, and how their peers might be curious or surprised. If your child is private and quiet, it will probably never be discussed. If your child is more open or outgoing, then it might get discussed but I doubt your child will be ostracized or teased! I actually think it's more likely to be an issue if you give your child the idea that it's something you need to hide or avoid talking about, because your child is more likely to respond to your attitude than their peers, especiallly at this age.