No I dint think you can 'help' the school find a replacement child. You'll just have to hope they do and actually be appreciative of the fact that if they do, they won't charge you the term of fees because most schools would charge you regardless - it is a very standard contract clause.
See it from the school angle - if your child doesn't go and everyone has made their choices of school now (it is nearly June) thry coukd have an empty seat in that class for 7 years - that's a lot if fees to lose out on - so a term of fees isn't nearly so bad and designed to stop people accepting multiple places thry can't actually attend. You e I ciousky had a change if circumstance but most parents would say that was the case if they thought it would save them the term of fees, so the rule has to apply to all. Think of it as saving you money in the long run as you won't be paying fees.
All you can do is write to the school again stressing how disappointed you are not to be able to take up the place and giving some detail of your circumstance change to hope they take some kind of pity on you, and also accepting in writing your obligation but stressing how grateful you will be if they find a replacement child which will relieve you of the financial burden to pay. The more co-operative and polite you can be the more chance of a favourable outcome I'd say, but you need to accept you are likely to need to pay.
I'd work on the basis that you WILL need to pay. You knew the terms and conditions and agreed to them. Schools have to enforce them to prevent people accepting multiple place and vanishing atbthisnpoint or later. So now you face the reality of the situation.
Remain positive and helpful to the school. Your circumstances may later change and you may want to take up a place. Accept you are financially committed - they aren't doing something mean and unfair to you. It's just unfortunate and really before you make a commitment and sign a contract like this you should be sure you have a term of fees accessible in case this happens.
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