I would stand up for the person being bullied. I don't think that anyone should be treated badly in school or out of school. Everyone should be treated fairly all of the time.
If you notice the bullying and do nothing about it this shows the bully that there are no consequences and he can keep doing it. Some things you can do to act against bullying is tell and adult and get the victim help. Standing up for the person bullied makes your school a better place and improves students experience