Kindergarten troubles!!

[Emrich]

My son is in Kindergarten and has recently started having trouble with a friend in his class.  The friendship started out very strong.  There is actually three of them who get along exceptionally well.  However, recently my son has come home crying more often than not because one particular boy says he won't be his friend, leaves him out, cries until my son gives him his toy or swing or whatever it may be that day and never shares with my son in return, says he has better things than my son, makes fun of his clothes etc.  I know this little boys Mom.  We sat at almost every basketball practice together, always seemed to get along well at playdate pick ups and drop offs etc.  The past few random times that there has been a problem between my son and hers (meaning I didn't act on every one) I have called her to see if we can work it out.  I felt stupid about doing it, but my son needed help and nice words, telling the teacher, etc. wasn't working and I didn't want my son to have to go and be left out and treated poorly everyday.  I called her the first few times trying to figure out if it was mutual or just my son causing problems or who is causing this.  Unlike her I am unable to watch them on the playground and go into the classroom because I just had a baby and I always have him and my 3 year old with me and cannot take them out in the cold.  I more thought we were becoming good enough friends to try and work this out.  Please note as well that on my son's report card his teacher put "he is a good friend to his classmates" and my little guy has never been in trouble at all.  Apparently her son has gotten into so much trouble that they have a lawyer (not anything to do with my son) also, the one time that we did bring it up to the teacher she wrote a note to me saying that the boys had to go to behavioral center to work out the other little boys problems and said nothing about my son doing anything wrong.  Anyway, my Husband dropped off our son at school on Monday only to have the other kid yell that he was going to so and so's (the third friend in their little group) house and that my son was not invited. Ok fine, it's fine that he cannot go to every play date and my boy knows that, but again the other boy was just trying to hurt my son's feelings.  And it worked.  He had a horrible day at school, couldn't stop crying etc. so I called this Mom once again.  I felt so stupid doing it but I really didn't know what else to do, I was thinking maybe we should get them together to talk when we were present, or try and work together to solve this so that noone gets hurt feelings, I have always been open to my son playing a part in some of the fights even though I didn't really think that.  She was so cold and rude to me.  She told me that my son needed to get over it, kids will be kids, shot back that my son is mean to hers, said that my kid is a liar, I mean was absolutely horrible.  She called me back after 10 mintutes and apologized.  Today at the spring program (20 kids and fewer parents-very small) she turned around saw me come in, glared at me and ignored me the rest of the time.  Just so happens that she is friends with kid #3's Mom, I thought we all were ok friends, but they both completely ignored me and then when another Mom (whom she has badmouthed horrible) came in she beckoned her over to sit with them.  I was so hurt, but more, I am afraid that my son will pay for my calling her.  Was I wrong to call her?  Should I just tell my son to suck it up and be tough?  This lady is a piece of work, I knew it when I met her (remember she has a lawyer for who knows what against the whole school board) and I would not put it past her to be ridiculous and immature.  What do I do?  I am trying to brush it off, but it is just bugging me and I am sad for my son because I already saw her be rude to him at the spring program (she told everyone in this circle good job but my son, which of course he noticed) This has gotten so far out of hand.  How would anyone else deal with this?