University of Geneva, Didactique de Sciences Physiques
Dr. Gasparini has developed and tested a course on GR for high school students (and accessible with conceptual and mathematical means at high school), see
http://www.nccr-swissmap.ch/education/highschool/GRcourse.
Students activities are available there, and a textbook was published in 2018
https://www.ppur.org/produit/876/9782889152094/Cosmologie%20%20relativite%20generale%20