The school could only start the investigation when the accuser wanders in to claim the OPs child attacked him - by which time he's had plenty of time to get his story straight and witnesses lined up.
ohgoodness, that is a complete assumption on your part. Even if it were true, surely the first action the school should take is look at the CCTV. Equally, your statement that the delay in watching the CCTV is an assumption on the OP's behalf is based on precisely no evidence whatsoever: what is your basis for that statement? The strong likelihood is that she says that because that is what the school told her.
Police are investigators and it takes them weeks to gather sufficient evidence to prosecute assaults.
Again, you're wandering into speculation to try to bolster your argument - I don't think you know much about police procedure. If the police view CCTV and can see from that that the person accused by the victim of assault was nowhere near, it really does not take the weeks to decide not to prosecute that person.
Then you have CCTV which might not show anything.
But this school has CCTV which shows that OP's son was innocent and which they didn't bother to view for three days.
Perhaps they're busy learning to analyse all the information and not leap to rash conclusions based on one side of part of a story.
They might be better off learning that the way you analyse information is by starting off with the most obvious source of neutral evidence that has the potential to resolve the case without the need for any further investigations - in this case, the CCTV.
You've assumed much and known little.
Oh, the irony. Your entire defence is based on the premise that the CCTV was inconclusive, based on precisely zero evidence. Why on earth do you think OP mentioned the delay in viewing the CCTV evidence if in fact it proved precisely nothing?
Suppose, just for the sake of argument , that it is indeed the case that they took 3 days to look at it and it clearly indicated OP's son's innocence: Do you view that as acceptable? Or are you so desperate to exonerate the school that your refuse to contemplate the possibility?