Nursery = Kindergarden (3-5)
Infants = Pre-School (5-7)
Juniors = Junior/Middle High (7-11)
Secondary = High-School (11-15)
GCSE = High-School (15-16)
6th Form = High-School (17-18) *Optional* (A-Level)
College = Community College (16+) (Job Training / GCSE / A-Level)
University = College (18+) (Degrees)
While we have SATs they are administered for seeding purposes.
Once when your 11yrs and again when your 15yrs, it generally isn't the same big deal over here.
Further more Universities are not like Colleges, in the fact that generally speaking you do not get a scholarship just for being particularly good at Arts/Sports/Etc.. usually it is because you require it as you cannot afford to goto Uni, but your marks have earnt you a placement.
Getting into a University generally means making sure the grads on your A-Levels are as high as possible. As you can only take 3-5, this makes it a pain trying to get into certain Universities.
This said, once your 16 and doing A-Levels, everything is elective. So you don't have to be stuck doing things you don't want to, effectively what you do at A-Level sets you on the path for your degrees. Each time you choose your specialising your subject further.. Generally speaking you can't really get high grades in English, Physics and Geography and expect to be allowed onto a Mathematics Degree.
The system we use isn't crazy or weird, remember this is effectively the same system in use for centuries; however the americans have chosen to alter thier original education system is up-to them.