The most likely resource for finding school logo stickers is through a school's website. These most likely have a separate section for these stickers, usually under auto or window accessories.
There are many online resources for finding school sticker logos and window decals. Two good websites are schoolpride.com and lids.com, which both carry a great deal of school paraphernalia. However, the best way to find a logo is through the actual school's website. Google the school's bookstore, and on the school website, type in "auto accessories." Search results from the school site should list any available school logos. When purchasing through the school site, the logo is sure to be authentic.
Many websites retail fake versions of school logos, often without having the correct or no copyright license to create the stickers. Thus, the logos do not quite look as they should. The safest option is to always go through the school first. If that is not possible, make sure to order the stickers or decals from a reputable website that has pictures of the stickers you are going to receive and holds a copyright license for the stickers.
Befoe purchasing, check to see if the colors match those on other paraphernalia. Also, make sure the image is high quality, not too blurry, too small or unrecognizable)?