My child is being bullied, or has been found bullying, at school. What should parents do?

Take both seriously: protect the child who is bullied and educate the child who bullies, because both are a parent's trust.
The Prophet ﷺ said that a Muslim is one from whose tongue and hand other Muslims are safe (Sahih al-Bukhari 10), and Allah forbids mockery and calling others by offensive nicknames (al-Hujurat 49:11). If your child is bullied: listen without blaming, write down the details, report in writing to the class teacher and the school administration, and follow up until action is taken; if there is injury or threats, make a police report. Watch for signs such as refusing to go to school, belongings going missing or changes in sleep. If your child bullies: remember the Prophet's ﷺ instruction to help a brother who oppresses by stopping him from oppressing (Sahih al-Bukhari 2444); do not deny it, look for the cause (for example copying violence at home or on screen), have the child apologise and repair any damage, and consider school counselling. See Educating the family.