Sketchup (installed on a school computer) or Sketchup for Schools (on a chromebook) is only recommended if you've had training.
The first step to 3D printing is designing the object you would like to print! You can do this in Tinkercad.com or by using the Sketchup application on one of our mac computers. (Note that sketchup is not a website, so you can't use it on chromebooks.)