Portland Public Schools and Tri-Met (local authority that runs the buses) let high school students travel for free. That's because PPS doesn't provide yellow bus service for students in high school.
One of my daughters did Tri-Met as a 7th Grader because it saved more than 30 minutes over the yellow bus. Taking the bus seems a lot less convenient than driving most of the time but for her it was faster. I took the bus with her the first day and she was fine after that. No bad experiences that I ever heard of.
When we were in Japan, we saw many students of all ages on the trains.