I stopped in elementary school.
At the time, it was because I was convinced that the pledge was essentially worshipping a false idol, and if I continued to do it, I would go to hell. Teachers couldn't fight that argument. Students didn't fuck with it either. I stood. I didn't cross my heart, and I didn't say it.
About 6th or 7th grade, I started challenging my "faith" and realized that the pledge was essentially swearing fealty to something that was supposed to serve the people, not the other way around. By highschool, I didn't even stand for it anymore. It was nationalism.