You need to record all of these incidents. It is more work, but If the school is to get extra support for this child, in a cash-strapped system, they need the evidence to present to show the severity of the situation. Ask the Senco for a format to use as tables such as ABC charts are often easier for others to read than narrative accounts.
Log any safeguarding concerns.
Ask the Senco to observe him in class or to teach the class whilst you observe. The latter can be enlightening as you may see incidents as they are starting to develop before the violent behaviour occurs and how he reacts to interventions. Check with Senco what referrals are being or can be made.
From your observations, look for patterns and triggers, e.g. Is it worse at certain times and is there a reason like he is hungry or someone else has stopped him doing something or is it better after he’s had a calming activity or done something physical?
Work with the Senco, and the family if they are compliant, to set targets and agree the strategies that are most likely to reduce the behaviours. There are clearly some unmet needs. These may be social, e.g. what is going on at home, and/ or physical or learning, e.g. if he has a hearing loss or receptive language issue, he may not understand what you want him to do, he may not have the social or language skills to get what he wants by asking appropriately.
If a situation escalates, you need to involve SLT every time. (Once he’s pinched the headteacher, you may find things start moving forward a bit quicker).
Before any of this, if the other children are unsafe then you need to decide what steps can be taken from Monday to change this. Again, record what you’ve done, either it worked, which is great, or it didn’t and adds to the evidence for the Headteacher and Local Authority of what you’ve tried.