wait wait wait! What! you wanna apply for Oxbridge next year? For 2016 entry or 2017 entry?
I don't think you can get into Oxbridge at the end of S5, no matter how capable you are. Unless you did AHs in S5..
Nuffield foundation can be done the summer before you enter S6.
I mean, it's up to the school. My guidance teacher told me if I take 3AHs and a H then I would kill myself lol, nevertheless, I took 3AHs and a H.
If your school is really against taking 3AHs and a H, then you take courses in college(if you want to). You get free periods in S6 anyway, so if you're very dedicated and motivated, then you can apply for college courses to do AHs or Hs.
And I mean, not many people do 7 Highers= = trust me. People who take 5 Highers in my school are the ones who are very academically capable. This is the case for the majority of state schools. If you want to do 7Hs then go to a private school. However, any university would look at your school.
Say you go to one of the worst state school in Scotland and you got 4As and a B in S5, the rest of the school gets a maximum of 1As or 2As, then it clearly shows you are very capable.
Whereas if you go to one of the best private/state school and you achieved 6As in S5, the rest of the school gets 7As/8As, then you are not a strong applicant.
However, Im not saying people who go to good schools are disadvantaged, as long you are the crème de la crème, you stand a good chance of getting into Oxbridge.