The trick here is to pick two broad (rather than very specific) reasons, which will give you the most scope to develop
The first reason is gendered differences in early socialisation
Fiona Norman (1988) found that most parents socialise boys and girls in different ways – they tend to be more gentle with girls, protect them more, and encourage them in more passive activities, such as reading with them, whereas ‘typical boys’ are encouraged to run around and ‘let of steam’ more.
Later on in school, this might explain why more boys do active subjects such as P.E. and why more girls do reflective, academic subjects such as English and sociology.
A further gender difference in socialisation is the toys boys and girls play with – dolls for girls and cars and tool sets for boys, which could explain differences in vocational subjects – health and social care subjects (working with children) are very female dominated, engineering (making and fixing) are very much male dominated.
However, Postmodernists would say that these stereotypes are breaking down, and that gender stereotypes in socialisation are much less common than in the past, hence why we are seeing more gender diversity in subject choice today.
Peer group pressure might also encourage boys to do ‘typically boys subjects’ and girls to do typically girls subjects.
This linked to hegemonic (dominant ideas about) masculinity – stereotypically, ‘real men’ are good at sport, and so boys are under pressure play sport to fit into their male peer group, this doesn’t apply to girls and could explain why more boys do PE later in their school careers.
Similarly hegemonic femininity also requires that girls ‘look good’ (as Louise Archer found) which could explain why it is mostly girls who do hair and beauty courses.
Verbal abuse is one way these peer groups reinforce dominant gender identities. Boys choosing girls’ subjects can be accused of being ‘gay’, and vice versa for girls, and this may steer them away from subjects which don’t fit in with their gender domains.
To analyse this even further all of this is especially true of working class girls and boys, and for younger children, less so for middle class and older children (doing A level for example).