Sadly, this fits the classic pattern of how schools 'manage' bullying.
You are in the first stage: denial. they will deny anything that cannot be proven.
If you later have one or more incidents which can be proven (by witnesses or physical evidence), you will move into the second stage where the school will play the whole thing down. Expect to hear phrases like "not a cause for concern", "a minor matter", and the classic "it's just an isolated incident".
If you can then go on to show sufficient "isolated incidents" are taking place, you will be treated to oh-so-sincere 'assurances' that they are aware of it and have it all under control........ you'll hear phrases like "......blah blah....robust policies.......blah blah...... measures put in place......blah blah-di blah blah".
Ultimately, they will find a way to turn the whole thing round and blame the victim for everything.
You have to be prepared to complain, and then to escalate the complaint through a whole string of defensive-minded vested interests: teacher, head, governors, academy trust/local authority, Ofsted (and be aware that Ofsted are reluctant to act over individual cases: they are only likely to be concerned if there are multiple cases.)
You are going to meet with denial at each stage unless you have facts, so log every single incident together with dates, times, places, witnesses, etc. Report every incident to school and keep a written record of their responses.
You will be seen at "the difficult parent" throughout, simply because schools cannot stop bullying. It is systemic to the whole institution of state schooling and, to a large degree, state schooling depends on it happening.