In most of England, your compulsory education starts in the first September after you turn four. You enter the 'Reception' class, and all your years in school from then are numbered from 1 to 11 (13 if you go beyond compulsory education). You begin in a primary school, which goes from Reception to Year 6. Reception, Year 1 and Year 2 make up infants (Key Stage 1), and Years 3, 4, 5 and 6 make up juniors (Key Stage 2).

After Year 6, you go into a secondary, or high school. You do your Key Stage 3 in Years 7, 8 and 9. You then begin to work on your GCSE qualifications* in Years 10 and 11, taking them at the end of Year 11. After that you can legally leave compulsory education, (you're 16 by then) though if you want to go to university then you'd usually go on to do your A levels in 6th form, which is the old and still-used word for Years 12 and 13.

Scotland has a different system that I'm not really aware of, and Wales differs a little in some respects from England. Also, there are some counties/regions that operate a 'Middle School' system where you go to three schools in your school life, a 'middle school' taking up the older children from primary school and younger ones from high school.

Schools tend to have different ways of functioning in terms of timing; the average day for all schools tends to be about from 9-3. Primary schools all tend to be rather fuzzy and different in how they time things, and secondary schools usually have about five lessons in a day.

To get into 6th form to do A levels to get a chance to go to uni, you usually need to get 5 GCSEs between A*- C. After that, you usually do three full A levels. Universities differ in what grades they want, with the more prestigious ones usually expecting better grades than lesser ones.

*Pass grades go from A* to G, with a fail grade of U. A levels have the same, but stop at E, again with the U fail grade.

In Bosnia and Herzegovina, you start going to primary school after you turn 6. There are 4 45-minute classes with 5 minute break between them, except after the second when you've got 15-minute break. It lasts 5 years.
After that you attend middle school. In middle school we have 6/7 45-minute classes with 5-minute break between them except after the second class when you've got a 15-minute break. It lasts 4 years.
After that you attend high school. Duration and number of classes and breaks are the same as in middle school.
Classes start at 7:30 am or at 1:30 depending on your shift.

After that you go to college if you want. Classes are held from 8:00am to 9:00pm, Monday to Saturday. So, our classes are somewhere in all that, sometimes in the morning, sometimes in the afternoon, sometimes in the evening. Sometimes you have a free day and sometimes you have to be at college all day, literally.

Grades are from 1 to 5 and in college from 1 to 10.

I think that's it.

How about kindergarden starting at 5 years old?