DS was badly physically and verbally bullied in Reception.
Teaching staff only said that 'he seemed upset', but because DS is very tall and looks older than his age, and his attacker was a tiny weedy chap, i knew that telling DS to hit back would simply lead to him being told off for bullying the smaller boy...
I taught him to yell 'STOP IT' at the absolute top of his voice whenever it happendd. Teaching staff got involved (apparently the first day was almost entertaining, as DS yelled perhaps 8 or 9 times, all justifiably) and it stopped.
As a teacher myself, I know how hard it can be to identify the 'original perpetrator' when there is an issue. It is often someone who is clever with their tongue, not their fists, and may combine a small amount of physical stuff with a HUGE amount of verbal needling. Sensible victims a) walk away, b) report what is happening (I have several boys in my class who mutter various code words to me on their way into class, alerting me to low-level things going on with their known 'needlers'), and c) use their voices not their fists to resolve issues.
I am a believer in consistent messages for children - if hitting is wrong, it is always wrong, even if someone hits you first. Young children do use their fists and feet to express emotions and responses that as adults we use our voices to explain and express. As adults, our job is to teach our children a better way of managing their emotions, not to teach them an 'interim' unclear rule that in fact it is fine to hit for the moment, but, oh, not unless you are being consistently bullied and not if they are smaller than you and not if it's an accident and not....