Standard route
In England, you school year cohort is based on your age on 1 Sept. You start Reception (year 0) on 1/9 the year that you are 4 years old, whether you were only 4 yesterday, or if you are 5 tomorrow.
Usually children move from Primary to Secondary school for Y7, ie the year they are 11 on 1 Sept.
To apply for state schools you apply to the local authority, using an online form. You put down number of schools in preference. The number of schools to put down varies across local authorities. We get 3, I’ve heard of some local authorities asking for 6 o 7.
School applications open on 1 Sept every year, and the deadline in mid Oct for secondary and early Jan for primary, but check this with the local authority. Offers are made in April.
Schools have priority criteria for admissions. You don’t necessarily go to your closest school. Proximity to the school can be very important. Faith schools can prioritise children christened in/practicing that religion.
Exceptions
Separate infants and Juniors Primary school - this is made up of Infants + Juniors. This is sometimes provided by 2 separate schools, sometimes on seperate sides, and requiring separate admissions the autumn before you want to start. Infants is reception, year 1, year 2. Juniors is Y3-6.
Some areas have first/middle/high schools. Our middle school is y6/7/8, so you apply in Y5, so you apply in the autumn of Y5.
Grammar schools - only a few grammar areas left. I’m not an expert. Some historic grammar schools retain the name, but are no longer selective.
(Sixth form is a different story all together).