Since it is asking for an evaluation of factors outside school you would discuss these for the larger part of your essay but also consider factors within schools for the sake of being balanced in your evaluation. Ultimately the question is asking how significant outside school factors are in infulencing the class difference in attainment. So you need to arrive at a conclusion as to whether outside school factors or inside school factors are more important, or if it is a combination of several.
Also, it is recommended to reference the item at least three times. You don't necessarily have to quote it but try to bring in phrases such as "As the item suggests" and "As explained in the item". For example in a paragraph discussing material deprivation/capital you could say "As Item A explains, the availability of material resources such as textbooks may influence pupil's attainment levels. Then carry on to explain in your own words.
I desperately need to revise so I can't remember any studies you could reference unfortunately.
Also I may have got some details wrong so please do your own research as well! I think I'm at least right in outline but I haven't done this in a year.
Some outside school factors:
Cultural capital. A concept developed by Pierre Bordieu. Basically argues middle-class children have more access to higher culture (literature, art etc). The opposite of this is cultural deprivation (Bernstein) which affects mainly working class children and describes exposure to popular culture (e.g. tabloid newspapers, reality tv shows) rather than higher culture.
Related to cultural capital are the restricted and elaborated codes of speech. Middle-class pupils generally have larger vocabularies which means they are better able to understand the language of school books, teachers and exam papers as well as expressing themselves more eloquently. Include this since it's mentioned in the item.
Material capital/deprivation. This is broken down into diet (working-class pupil are more likely to eat unhealthily and miss school due to illness and have poor concentration), housing (damp overcrowded housing can lead to illness and there may not be any space to study in for working-clas pupils), affording (or not!) additional resources such as educational trips, textbooks and even the uniform, and affording higher education after school. While studies look into the effects of material deprivation on the working class, since this question is about middle class pupils it might be necessary to invert things a bit. So, "Middle class pupils can afford x" instead of "Working class pupils cannot afford x". Definitely write about material resources since it is mentioned in the item.
Cultural differences - higher aspirations in middle-class families, higher parental aspirations and involvement, deferred gratification, a willingness to put hard work in for the future's sake. The opposites may apply in general to working-class families. Though an important point of evaluation here is that these theories may rely too much on stereotypes of the different classes?
Then also consider inside school factors - you are evaluating whether these have as much impact as outside school factors. I've listed a few but you don't have to go into quite as much detail as with the outside school factors as the question doesn't name these directly. You might not have time to cover all of these.
Effects of marketisation
The hidden curriculum benefitting middle-class pupils
Symbolic violence against working-class pupils
As the question asks you to evaluate, every time you make a point you need to test it out. Does it sound plausible and is there any other evidence to agree with it? Or is there any evidence to contradict it? If you have referenced a study, are there any flaws in the research method such as a small sample size? It is essential you do this to fulfil the requirements of the question.
Then in the conclusion you should reach a decision as to whether outside-school factors are indeed the main causes of middle class pupil's higher achievements. Use objective language by saying "One thinks" as opposed to "I think".
I hope some of this helped.