Poop! Sanders what utter crap! 
The mum who called the police is cookoo!
OK here is my off the top of the head thoughts, feel free to ignore.
- My birth dd had some problems in year 3 and one thing that helped us a lot of was inviting kids for play dates and finding the things that helped her to make connections. My dd is dyslexic and needs 'props' to hang onto, toys, puppets etc. She finds it easier to make friends when she feels she has something there. I remember a nervous teenager at a Christian youth camp 30 years ago saying she was nervous but 'all packed up' - she had packet of cigarettes with her! We are the same with our big coffee mugs/G and Ts or whatever. I understand it more now but have spent years trying to wrestle large toys off dd before Brownie camp or cinema trips!
Any way....
One of dd's friends arrived for a play date with an 'alien baby' in 'slime- filled egg'!
think this
I was horrified when dd said she wanted one too, but we got her one and then another, and we found out that in year 3 (last year, my dd is same school year as your little one) they were all the rage! Suddenly, she had a point of contact with the other kids and having that 'toy' (and we bought many more) was great. they looked after each others alien babies and even took each others babies home for visits etc!
When dd had problems I invited girls who I felt would be 'nice' home for play dates and I managed to average one or two play dates per week from asking her who she wanted to invite back. I just typed out or wrote out a short note with the contact details and gave them to the child in front of their mum with the words 'Little greyhound' would like to invite you over to tea/to play etc would you like to come?' and if they said yes I said when are you free.
We kept play dates short, only included tea if we felt it was right and my advice is to make sure you end the play date by returning the child at a set time rather than waiting for other parent to collect them. If parents are late and play date is all going pear-shaped, it can be excruciating. I have experienced this so I know!
I would prepare your dd for the fact some people may not want to come back and you can 'incentive-ise' the other child with words like, 'we could decorate mugs/have ice cream sundaes/make pizza etc whatever you like.
This could be children in her year or below or a mixture.
I would also suggest if you really think the other child will not want to come then speak to their mum without your dd there.
This is hard, to do and hard to meet up with them appearing casual while secretly thinking - YES PLEASE COME FOR A VISIT NEXT WEEK! Harder still as kids get older and may meet their mum at the gate or even in some cases walk home alone.
Pick dates not too far ahead but not come back now as that could be intimidating for your little one.
I am sure you know all this, I am sorry if this sounds like I am trying to each anyone to suck eggs!
It is hard and it is difficult if you feel dd will get more rejection but you probably do need to give it a go to help her make better friends - either in her year or in the year below or through a local club (brownies/church club/swimming or whatever).
Make sure she does understand for the play date to work the other child needs to enjoy themselves! My dd wanted people over but then when they wanted to go upstairs to play she wanted to watch TV. I now allow some TV and a snack and drink at start of play date and then insist on outdoor or indoor play with no TV. Having a big (safe) netted trampoline is a big hit with friends. Hers not mine!
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Build up her confidence, teach her skills, I know you are doing it already! Empathise a lot, it is so hard to make friends. And maybe put yourself literally in her shoes, make the move to ask one of the more friendlier looking mums from her year or below out for a coffee. Then you can report back, I felt quite nervous inviting Mrs Blah Blah over for coffee but she was nice and Kristy/Megan/Charlotte/Patty/Francesca sounds lovely so let’s meet her in the park for a play etc.
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Lastly (or maybe FIRSTLY), school need to know your child is a priority for you, I am sure they do!
So whatever else is going on with other kids to ensure your dd's problems do not escalate they need to meet her needs.
Just for the record I am an older mum too and one of the younger mums actually said she prefers older mums as she finds them less judgemental! So do not assume all the younger mums will shun you.
It is hard but if there is a point of contact, another child struggling with school work, struggling to make friends, another child taking up a new sport or still learning to swim etc then it is a chance for your dd to help someone else (we can work on this together etc). The best way to get a friend is to be a friend!
Corny! I know but you just need to find those little ways. It’s probably easier now than when she gets to high school so try and see the positives –
This has come up now, and not when she is at high school.
School are trying to be supportive even if they do have other priorities (which you would expect, of course)
This has uncovered the problems in this friendship and having been in a triangle friendship (twice) it is a bit of a pain if friendships are not even – I feel often triangle friendships are not a clear cut all three of us like each other equally (as kids I mean) it is usually one girl (or boy maybe – no idea with boys) in the middle and other two (or in one case three) of us all dancing attendance on one girl!
Good luck, keep us posted and I really hope all will be well.