I used to work in an education safeguarding role and I can second that traumatised kids get things confused all the time- as @tonyhawks23 and @Jellycatspyjamas said, trauma does stuff to their memories and narratives that mean certain things get muddled or confused.
School have to investigate everything though, and it can be helpful to have a discussion with yourself and your partner about how you react when (not if!) you are contacted, because it can send you into a tailspin when you get the call. We treat it like any incident of poor behaviour- so we give our version of events if we know, say that we will have to speak to our other half if we dont know, but then ask for more details about the context of the disclosure- was he with friends? Who did he disclose to? What were they doing? What exactly did they say? Maybe they have a tendency to exaggerate when talking with certain friends for example, or dislike PE and know that saying soemthing on a Thirsday means they get to sit in an office and chat with a learning mentor rather than going out into the cold. Sarah Naish talks about one of her children being experts in appealing to get an adults "sympathetic face" because that is how her children survived their early childhood. This is really relevant because it tells you your child is triggered and feels unsafe and needs to appeal to adults in whatever way they can.
I also think it is important to know a bit about the process of safeguarding in schools to reassure yourself about any outcomes.
it may be different in your school, so do read the policy, but most schools ive worked in do something like this:
The child makes a disclosure. Teacher has to report it, even if they know it is unlikely to be true, because it might just be and they are not trained to make that decision. So they do that, the child is usually taken aside by another member of staff, usually the DSL and asked to tell again what happened. At this point the story either disintegrates or consolidates, but the line of questioning is very specific because leading questions aren't allowed. Based on what they know of the child and the situation, they will investigate, a part of which is getting relevant peoples version of events before making a decision about what to do next, which can include calling parents, speaking to siblings, discussing with the teacher about the context of the disclosure etc. From what brought are telling me, you have been involved at this stage, the information gathering. At this point the DSL will make a choice as to what should happen next.
In our area at least, the DSL would then have to be concerned enough to make a referral to MASH, and the first question that is usually asked is "what do the parents say". Most SG leads will have already spoken to the parents unless there is specific prior concerns because they know that question is coming.
At that point MASH will advise- it can vary in what could happen. It may be that they tell the school to say specific things to the family and signpost to resources, they might recommend a referral into early help programmes, arrange a visit from a social worker in the future, immediately send a social worker to speak to the family before the child is released into their care, or send social workers and police if a possible crime has been committed. (Im sure there are other outcomes too, but those are a few ive known to happen) It is a sliding scale, but trust me, i should imagine that if the DSL has any experience at all, they will have been involved in all of those at some point. None of those things are pleasant, or wanted, but knowing the possible outcomes always helped me keep it in perspective.
Obviously, not everyone involved in the process understands how trauma works, but it can be a very useful opportunity to advocate for further support for your child. I remember reading one person on FB explaining how her child made such allegations that were always found to be untrue, but after sitting down several times with social services and listing all the interventions her child had, how she parented and the challenges of the children, she was expedited through the CAMHS process and, bizarrely, had her child referred to a horse riding outreach programme. She took whatever they would offer her basically.
We have found it helpful to preempt lots of this with our LO- we have regular conversations on the school gate and with the DSL to ensure that we are all on the same page about what he is saying and how that can be interpreted. Nursery have noticed similar patterns, so if he says something about a teacher, I always speak to the nursery teacher and let them know he has said it so that they can speak with him and remind them that i trust the investigative process of the school, and offer to write up what he has told me in an email if they need it. I shared some phrases that we use with him when we know he is misinterpreting the situation and it has helped. School feels supportive, but we have had to lean into it and trust the key people in school with maybe more information about our parenting and personal lives than I would have expected.
It is a tough old road to tread, but as someone just starting out, I understand how scary it can feel to get that call. Try to see it as your child being scared and confused, and surviving how best they can, and then move through it with the professionals in that way if you can.