well for CIE subjects ( bio,chem ,phy) which need practical work so u need to be in school to practice the practical as u will do the practical exam by yourself under the supervision of the examiners also u have to take a look on the syllabus to know the route which u're going to take through different papers and what u need to complete as-level or a-level
for Edexcel the bio, chem phy ) have an alternative to practical papers so u won't need a school if u're a homeschooling student but keep in mind that u should search for the practical to be well prepared for it
a difference too is the exam sessions for CIE its oct/nov and may/june
for edxecel its jan and may/june
that's what I know and if I got any other info. I'll post as soon as I can isa