New Preschool & fighting

Hi Ladies,

Hoping for some advice here. My DD3 started a new montesori preschool about 6 weeks ago and absolutely loves it, problem is 2 weeks after starting another little girl started and they became friends, at first I thought this was great. She told me a few times that xxx is not my best friend any more!! but then the next day she would say they played etc. Around this time DD started hitting / slapping DH, DS and I and I noticed a lot of cheek. It was only yesterday when the director mentioned they have been having some terrible "scraps" slapping / hair pulling / scratching etc. Now DD has been going to day care since she was 8 months old and I have play dates all the time with friends kids / mothers group etc and I have NEVER seen / heard of her hitting another friend. Further to this, she has slept through the night since she was 5 weeks old, only waking ONCE because she was very sick, otherwise she has never woken, except last night where she woke at 2.30am calling out for me, I didn't hear her as I was with DS settling her, and as DH was in the spare room (trying to get some sleep he heard her and went in, she wanted comfort and pats on the back). she has been very clingy with me (which she never normally is), so this morning I spoke to one of the Misteresses and the director and asked for them to be separated. There is an opportunity to swap her days from w-f to m-w. I feel that DD is only friends with this little girl because she has not made any other friends as yet, and I don't want her to grow up thinking she needs to "settle" in relationships. Sorry for the novel, but any suggestions what I should do... So far I all I can come up with is:
* Change days - is this a good idea?
* Encourage her to make new friends - but she keeps finding her way back to this little girl
* Reinforce hitting / scratching is wrong - which we do, but doesn't seem to sink in (any other ideas welcome in this department)
* Trust the DC will separate them
Other than that, I know she loves this centre and is really thriving in other areas. So really look forward to your feedback as it makes me so sad to see her like this!