Absolutely not. There is never a time when it is okay to bully.
What may begin as a seemingly 'harmless, kids will be kids' kind of challenging another child, evolves oftent times into something more serious.
I taught children for forty years. I did not 'miss' any overtures of bullying when the children were in my midst. However there were times during the day when the children were not with me. There were opportunities for children to pick on others. I always told children to inform me if someting was happening when they were not with me. Sometimes they were threatened into not telling; 'if you tell, I will beat you up.'
Definitely if a child complains of being bullied, the parent should take action. Contact the school teacher immediately if a report of being bullied is reported to them. If the child continues to complain, then the parent needs to contact the principal. Nip it in the bud early and put an end to it.
Too much tragedy has resulted from ignoring seemingly 'harmless' remarks or actions taken by a student on a student.