I'm not sure about "upstanders" against bullying in and of itself, so much as a matter of extremes... for example, when there's physical abuse or fighting. When I was coming up, the lesson was, "If you have the ability to intercede, you have a responsibility to intercede."
By the time my son was in elementary school, it had become, "Don't get involved, get a teacher or call the police."
I think when it comes to non-violent bullying, that people should encourage and empower the bullied to stick up for themselves... and should it become violent, then intercede as necessary.
I was taking a class on geoengineering and was partnered for a debate with a very justice-oriented student who had spent a lot of time in the Amazon rainforest learning about and who had brought me to a great lecture about the conservation of the rainforest so we'd done a lot of preparation together. I was playing a sort of Henry Petroski character who was a right-wing engineer who would get accused of wanting to build a Dyson sphere. [1]
The day before our debate he and his friends were at a karoke night and his friend tried to intervene when a stranger was bullying another stranger [2] in the sense that "you just don't do that". His friend got stabbed and my partner rode up to Syracuse in the ambulance with him to support him and couldn't come.
I wound up giving both sides of the presentation which fortunately was easy because I had his slides and we'd done enough prep. But yeah, this points out that intervening is a dangerous thing.
When I was in college we had a student, Chris Cater, who was waging a war against gays and had polarized the community around him and created a number of circles of support, for instance some people who were involved in violent attacks, the baptist student union who "loved the sinner but hate the sin", pot smokers who liked that he was criminally minded and was the best dealer because he'd take risks others wouldn't, etc. When he and a friend of mine came by the college radio station and were preparing to prick a can of shaving cream and throw it into a meeting of "gays and friends" I told my friend "Dom, that's dumb, that's really dumb." I didn't see it so much as "taking a stand" but just my spontaneous reaction.
It was the beginning of a whole lot of trouble because Cater and his gang would ambush me at my dorm (stopped the time I ducked into the dorm of the captain of the Rugby team) and when I was out walking. It put a lot of pressure on me because I wasn't really that gay positive at the time and was a stress on my relationships with Christian, pot-smoking, and other friends who found something positive in Cater's charsima and I felt even I was in one of his outermost circles of support and felt the accompanying moral injury because I just didn't want to be involved or seek connections with other victims.
I'm still angry at the school administration because they were slow to intervene and I blame them for the suicide of two students who I think wasn't so much motivated by the attacks but by the failure of institutions to respond -- they don't tell you your life is worthless, they show you.
So yeah, taking a stand can really turn your life upside down.
[1] I tend to become what a group is lacking. Put me in a left-wing group and I am the rightmost, put me in a right-wing group and I am the leftmost. Not so much like I'm a centrist but I think if a class in the Engineering school doesn't have a right-wing engineer in it or somebody like a character in a Heinlein novel something is wrong.
[2] ... I think a continuation of a wider conflict, possibly gang-related