How to make a 3x3 grid in Illustrator?

to stay within the diabolical rules,

You could make an art board say

3 inches by 3 inches

Set the rulers to inches  (right click  the ruler if needed)

Click on Artboard tool,  - which will constrain the rulers just to the artboard-

And drag rulers to 0,1, 2 and 3 inches

both horizontally and vertically..

then make the artboard bigger to show all edges

(Method stolen from Monicas 'Dartboad' answer last week ..)

ebony,

As I understand the school task, you are not supposed to use a solution like the rectangle > split into grid, but something indefinite based on the rulers.

Are there no hints about the specific meaning of the requirement?

creating a 3x3 grid using the rulers

Draw a rectangle

Object > Path > Split into grid.

Set up the grid you need and set the option to create guides.