When setting up a new year group and possibly a new form group in order to set up registers. You should go to Admin>Config>Core Setup>Years and enter -1 for the year below reception (or -2 for the year below that).
If a new form is required, you can set it up under Admin>Config>Core Setup>Forms.
You will also need to ensure your lowest year group is correct in Admin>Config>Core Setup>School.
You will also have to check meal types are available for the new year groups to select, if meals are provided in Admin>Meals>Set Up Meal Types, 
as well as,
set up the classes for assessment purposes in Admin>Classes. 
Once this has been set up you can assign new students to the appropriate year and form group. There will be additional information to be entered on the student's extended tab, including hours at setting and funded hours.