Usually no, there's been opportunities, but I don't like confrontation that much and I prefer keeping to my own business. Especially with how other students in my school enforced a huge "snitches get stitches" policy.
One time I was called out of class and questioned by a teacher about a bullying incident, which I didn't say much. The reason he was getting harassed was due to getting another student in trouble. Every time they called a witness out of class, this kid and his friends would later confront the person to make sure they didn't tell anything. Thankfully I did not, while it may have been the right thing, I still did not want to be attacked by 5 other kids.