God Bitter thats a nightmare 
We were lucky in that a lot of other parents had had problems with this one boy that was bullying dd. They tried to fob us off at first but once I think the 6th parent had gone in they had to take notice.
Our teacher was crap too, we found out later shes friends with this boy's parents 
Anyway, I'd go in and state your concerns, explain in the severest terms how its affecting ds out of school, ask to see their bullying policy (or get a copy in advance off their website). Ask them to start the bullying policy on these kids now (if they havent already), ask them if they've talked to these kids parents too.
If they say they haven't seen any incidents then you need to discuss why there isn't more supervision at lunch/playtimes.
Main question is 'How are you going to convince me my child is safe and happy while at your school', get them to tell you what they are going to do, its their reponsibility to keep your ds safe in school.
We tried to be reasonable by saying 'they're 5/6 we know there will be playground scuffles and upsets, but this is far beyond that and needs to be nipped in the bud now'.
What our school did (eventually):
- got the parent's of the child in repeatedly
- used punishments such as no football practise with him, he's now on a written warning (leads to expulsion eventually)
- introduced much more supervision at play time /lunch
- in 'circle time' discussed being 'kind' etc, also asked children to discuss their feelings (this only works if the children arent too frightened to say obviously)
- had lots of conversations and discussions with all the children all the time about 'personal space' so when they line up they have to leave a 'person' space in front and behind, at the end of each day they have to sit on the floor on their own in a 'space' for 2 mins to try to explain boundaries and inappropriate play
- we had an original meeting with the teacher and then a meeting every friday pm for 2 weeks till we felt better about the situation - communication is really key to making you have confidence in their ability to deal with the situation.
- discuss the idea of a friendship stop or buddy bench with them
- at our school there are monitors, older children who also check everyone is ok and help the younger ones if they see bullying etc, they will tell the teachers off any incidents too.
- they brought a thing in where each child was paired with a different childs for say 3 days, then they swapped. This broke up some 'unholy alliances' a bit and got kids talking to each other/becoming friends with ones they hadnt mixed with before.
will carry on thinking....
write questions down before you go in maybe so you dont forget anything you want to say.