You can come whenever you want to (or when you are asked to, of course). 
There are some grammar explanations and some exercises on your level. 
In addition,  you can ask for your teacher's help. 
So... you just have to select your level and go on!

Level 1       Level 2       Level 3       Level 4       Level 5       Level 7       Level 8

Crescer Sempre        Level C        Level D        Level E        Level F        Level H   3rd Grade