Good answer ubasses, and it covers off both the IHT issue, which I and others alluded to, but also how to get around the issue of the money being seen by HMRC as income for the OP's son.
Its a really interesting question and I spent a little time researching this today, but couldn't find anything definitive in HMRC files about when a 'gift' (that I think we all agree is not income for tax purposes) becomes a 'payment for services (either explicitly or implicitly)'. Most of us, I trust, would agree the later is taxable for the recipient.
By making a specific statement that it is a gift to express pleasure for services done only, that probably nails it. I don't think any reference should be made about future expectations of volunteering.
The last thing I comment is that the very charitable lady would not be able to claim personal tax relief on this gift, because it is not going to a charitable beneficiary, whereas the money she donated to the school (probably) is. She may not be bothered about that however.