When I worked at a very old elementary school, I found a trove of old school paper from the 1950’s. I have scanned them as jpgs. You could use them as backgrounds or to make other school materials.
The notecard and the blue and pink kinder lines are not letter size. The brown kinder lines with open space is larger than 8.5 x 11, but it can easily be scaled down. The rest of the papers are 8.5 x 11.
You can purchase these at Teachers Pay Teachers or Counselor Created.