At the school I work in, a student told another they should "go away and kill themselves". The student who said that was permanently excluded. Things like that are very serious indeed.
This country has hopefully learned lessons from the whole Rochdale thing where people were unwilling to deal with the situation for fear of appearing racist, so while these boys' bullying may not be racist ('just' bullies picking on the more vulnerable for no reason other than that they are vulnerable) the school still need to deal with it. Bullying is bullying no matter what the motivation.
Document everything. Finally, if nothing is done, ask the school, and the County Council, about a 'managed move' to another school. Sometimes there's no point sticking around to be bullied some more. Some bullies never change, so your only recourse is to move out of their radar.