Yes, you go from a very rough approximation of science, which is sufficient for the vast majority of people to explain the everyday phenomena they encounter. To theoretical and abstract science that enables degree level study.
For example, in GCSE chemistry we learn atomic structure, and even the history of its discovery, up to the Bohr model of a positive nucleus surrounded by electron 'shells' in the 2,8,8,8 pattern. This explains most of the reactivity most of us need to know.
At A level, you then discover that actually, there are sub shells: s, p and d shells, and the periodic table can be used to work out in which shells electrons are likely to exist (also, we've gone from the concrete "electrons are here" to he abstract "electrons can theoretically be anywhere, across the room from their nucleus even, but there is a greater likelihood of them being in one of the orbitals"), these electrons also behave in certain ways, you learn about "spin" and resonance.
I'm rambling. I need to get my head back into teaching mode tonight! Thanks!
It's just such a huge jump, most students find it hard. But we now have the Internet! And these sources like Khan Academy will effectively teach and re-teach bits whenever you like. Most importantly students should ask their teachers and keep on asking till they understand.
Another common problem is that the sciences at this level are more often taught in tertiary colleges, in a lecture format, by very clever teachers but who may not have taught younger students, and so use fewer different strategies to support learning. Whereas I'd make models and use discussion and questioning and group activities, a tertiary lecturer might simply... lecture.
hellsbells Khan Academy (Salman Khan is a legend) covers lots of physics too. I'll have a look for some physics sites when I pretend to do some work later and post any more links I find, although I don't know much about the physics A level.