In my opinion, typical chairs and desks can be excluded since it may not affect significantly illuminance level. However, you may consult with your daylight consultant or modeler for further discussion. If you can include all furniture, it will be better.
To increase the accuracy, you may even try to adjust your grid position according to the height of typical desk in the classroom, but your reviewer may not accept that since the reference mentions the grid shall be at 2'-6".