The Swedish school year is split into two terms/semesters with breaks inbetween. The first term, beginning the school year, starts in mid-August and ends a few days before christmas. The spring term starts in early January and ends in mid-June.
One school year is two halves from normal years. A school year is therefore referred to like "the school year 2023/2024" since it streches over halves from both years.
You can start preschool from when you turn 1 year old, but it is not compulsory.
From the autumn of the year you turn 3, you have the right to public preschool. Public preschool covers 15 hours per week and is free of charge.
Starts the autumn of the year you turn 6 and it is compulsory. I was actually surprised to learn that preschool class is separate from primary school, but now that I think about it, it really was just preschool activities but in a classroom...
Primary school is the longest phase of the school system with a total of 9 school years. Together with preschool class, Swedes have a total 10 years of compulsory school.
Primary school is separated into low stage (lågstadiet), middle stage (mellanstadiet) and high stage (högstadiet). Each stage is separated into three grades.
Low stage is grade 1, 2 and 3. (7, 8 and 9 years old)
Middle stage is grade 4, 5 and 6. (10, 11 and 12 years old)
High stage is grade 7, 8 and 9. (13, 14 and 15 years old)
Google does not give me one straight answer to what this should be translated to (High school? Upper secondary school?) so I will just call it by its Swedish name.
Gymnasieskola, colloquially just gymnasuim, is a free, optional secondary education for those who have completed primary school.
The special thing about gymnasium is that you get to choose your orientation/major. You go to gymnasium for 3 school years, these also separated into grade 1, 2 and 3. (16, 17 and 18 years old)