We need to understand how the education system in England is different from the education system in the United States. In the US, you guys start in Pre-Kinder, and then go into Kindergarten, then Grade 1 and so on. In the UK, our first year of school which is equivalent to Pre-Kinder is called Reception. Then we go into Year 1, which is the equivalent of Kindergarten. Our Year 2 is therefore the equivalent of Grade 1 in the American system.
Now, if we look at the novel, in the chapter entitled "Miss Honey," we are told that Matilda, although she starts school a little late, is put into the bottom class:
Naturally Matilda was put in the bottom class, where there were eighteen other small boys and girls about the same age as her.
This indicates that Matilda was put into the Reception class, which is the equivalent of Pre-Kinder in the American system.