Just to add to @ucme‘s above (which made perfect sense to me, but may not to those of you quoting school years rather than ages)...
Age 4 – Reception (pre year 1)
Age 5 – Enter year 1 of Primary School. Years Reception – 2 are known as infants.
Age 7 – Enter year 3 of Primary School. Years 3–6 are known as juniors.
Age 11 – Enter year 7, now at High School (aka secondary school). Years 7–11 gets you to the end of compulsory education, finishing aged 16. Exams are taken that stay with you for life.
Age 16 – Option to enter year 12, for 2 years of what is still traditionally known as “Sixth Form”. (Years 12–13 = Sixth Form. This is a throw back to when the year numbers re-started from 1 in secondary school. Interestingly there never was a 7th form.) These 2 years might be done still at secondary school if it offers it, or you might go to a specialist Sixth Form College. Exams are taken, that stay with you for life.
There are also technical colleges that cover these two years (aged 16–18) where you can study practical based subjects (such as chef training for example).
Beyond Sixth Form, aged 18, you can choose to go to University, where you typically study for 3 years. (In Scotland this is 4 years.) We don’t have different names for the different years of university, other than 1st 2nd and 3rd (though we’d typically refer to this as the Final year I suppose). University is where you graduate from. (We don’t “graduate” from school, we just do our exams and move on.) Having successfully completed your university course you have a degree.
Studies beyond your degree are called “Post-Graduate”, and depending on what you do, for differing lengths of time, you can achieve qualifications such as a Post Graduate Certificate or a Masters, or a Doctorate.