This does sound like it might be bullying; I'm so sorry. Definitely talk to the teacher, and don't be satisfied unless your daughter starts to be happier, because sometimes it takes a lot of trouble to make a difference. This is true for both the social problems and the dyslexia.
It is very common for dyslexic children to lose confidence and to hate doing whatever special activities the school organises for them. You can work to show her that dyslexic doesn't mean stupid (Einstein, Picasso, Churchill and John Lennon were all dyslexic for instance.) www.dyslexia-test.com/famous.html
Lots of children don't have a best friend, but you can help her friendships along by inviting other kids over to play. Maybe the teacher can suggest a few friends who seem compatible, if your daughter doesn't know who she wants to ask. I have found that inviting the same girls over between school and guides is a great way to cement friendships.
I don't mean to be intrusive, but if your daughter is actually overweight (I realise the teasing is probably not based on anything real), I think that does make life harder in school. If you think she might have a weight problem you could start another thread, or look at an existing one.
The fact that the school is helpful with dyslexia is a great sign, i think; they will probably be really helpful in other ways too, and since it is a small school, they will already know your daughter well.
I hope she feels better really soon.