Hi C.,
This may be resolved by now, but figured I'd offer a few thoughts.
I'm a preschool teacher/former toddler teacher/nanny, so my view is this:
it does take a long time for some children to bond with the teacher, and I'm not surprised to hear it's "so late in the year". We can all work together to develop a daily rhythm, but the intense bonds may show themselves much later in the schoool year.
One thing I would suggest, if this hasn't been resolved, is that the teacher do some planning with the co-teacher to keep your son directed and busy at these times, and to work as a team to help your son through these moments. My suggestion would be that if he becomes clingy and upset, the teacher should give him a short "I've changed your diaper/helped you potty, now it's Sally's turn. Teacher Jill (the co-teacher or assistant) will help you now." Here, she acknowledges his desire for her attention, and then redirects him, so empathy is given but she is not further engaging with him.
Or, when friends are hurt: "Did you see Sally fall down? She is so hurt and sad, so I'm going to hold her for a while. Would you like to find a toy for Sally to hold/would you like to walk with us to get Sally an ice pack?" (in this second example, he is being given an opportunity to focus on his hurt friend and help to them feel better/ to understand that others feelings are important.)
At both these times, the coteacher should be cued in that your son will need to be brought into the group activity again, or he can choose to sit in a cozy corner and look at books, or find another activity. She can gently assist him away from the Teacher, and help him get engaged appropriately.
Yes, this is likely an attention-getting behavior, and the teachers will have to decide how much attention-- and from whom-- they want to give this. If it were me, I'd plan a meeting with my coteacher for just this purpose, to make some plans in advance so we could work as a team to help your boy. I believe that an empathetic response ("I know you really love Teacher, and want to play with her. She's helping other kids now, just like she helped you.") and then move to the distraction/redirection piece of the problem-solving by getting him re-engaged. This could even be a "letter" to the teacher, taken by the coteacher. "I see you want to talk to Teacher, and she's helping Sally. Let's write her a letter. What do you want to tell her? 'Dear Teacher, I miss you...'" I've found that letter-writing with young children is a very concrete way to acknowledge their emotions. It's very empathetic and helps children move on from their upset to feeling understood, and then able to move forward in their day with play.
Hope that helps.... and on a side note, please do "surf" the sea of estrogen here. More gender balance on this site never a bad thing.