With regards to the school, the fact it has been intimated this is a legal matter is a bit of a gift. If they make contact advise them that all communications must go via your solicitors.
I would disagree with this. If OP's son is sitting A-level exams this May/June, then her first priority will be trying to get the school to allow him to sit his exams at the school. Sitting exams as an external candidate is difficult and expensive, and the closer you get to the exam period, the harder it is to organise.
If the school decide to exclude, they hold all the cards in this area (to some extend). Depending on what has actually happened, there may still be an option for OP's son to sit his A-levels at the school for free with various arrangements in place.
Achieving this for her son should be her priority, and if she starts going down the route of "all communication via solicitors" it will be relatively easy for the school (assuming they do decide to exclude) to "communicate" that he cannot sit his exams on their site.
OP's ideal is presumably, actually, for her son to not be excluded. To achieve this, it is very likely she will need to work with the school- especially as the time frame before exams is really relatively short.
For him to be allowed to go back to the school, the school will have to feel confident he is not a threat to any of their younger students, and this will likely involve some show of remorse/responsibility from OP's son (obviously depending on what has been said etc.), but hopefully this can be done verbally.
FWIW, I agree writing a letter of apology would be a bad idea for lots of reasons.