In the British education system, you can study either single science, double science or triple science at GCSE. In each case, you study biology, chemistry and physics and take exams in each, but if you take single science you receive one GCSE, if you take double science you receive two, and if you take triple science you receive three (one each in biology, chemistry and physics.) Single science consists of so-called 'core' modules, double science is the core plus 'additional science' and the triple is those two plus some extra modules. I can't remember the specifics, it's been seven or eight years since I sat those exams. I did triple science at school by the way.

Gary is right. If the schoolgirl is talking about the GCSE exam subjects she's taking, 'Double Science' is a course leading to two qualifications rather than just one.

I think that's a British (as in Britain, rather than British English) only phrase. I had always thought that it means that they have a two-hour class, rather than the normal period. But I just did a google search, and found that it may be study that leads to a double credit: https://en.wikipedia.org/wiki/Double_Award_Science That may mean that they have a double period as well...

Iris, while I can't give a firm answer without more context, my best guess is that the speaker means to say that she is taking two science classes.