I'm very fortunate that the staff at DD's school do an awful lot of extra teaching after school - there are clubs for a variety of activities as well as extra maths, reading and phonics. Take up is extremely high for the classes - some the parents can join too, the children can also go to after-school club until 5.30 once the club or class has finished if childcare is an issue.
I imagine that the pupil premium money helps pay for some of this and the school gets a lot given the intake.
I very much doubt that they are eating biscuits in the staff room, but if a school has poor results then parents are entitled to ask what the reasons are and what strategies the school is following to remedy the situation.
Having been a school governor for a secondary school that had a fair number of children with major issues of the sort you mention, I'm not unaware of the struggles that many children and teachers faced every day and that often it is a losing battle no matter what is tried, or how caring and committed the staff.
Also agree that immigrant families are on the whole very supportive and appreciative of education.
The school that shocked me the most was one my sister's DC attended that was 100% White British, with very low levels of FSM and SEN, in a nice town and yet only managed 56% getting L4 in reading at KS2. I really couldn't see what the excuse was there.