Is there a UK charity which rebuilding schools - particularly primary
schools - in Pakistan?  I am charity coordinator for our school and I
would like our boys to get involved.  I read an article about some
such organisation which quoted £600 to build a classroom and £6000 for
an entire school, using local materials and labour, but I have mislaid
the details.

Dea Bede, 

Here are several that work on rebuilding schools in Pakistan: 

Save the Children
<http://www.savethechildren.org.uk/scuk/jsp/wherewework/country.jsp?ukww=ww&section=sctrlasia&subsection=pakistan>

(see also: 
Pakistan: Education Key Priority in Quake Region - Save the Children
<http://72.14.203.104/search?q=cache:DMS5rMNDi1AJ:www.crin.org/resources/infodetail.asp%3Fid%3D7383+charity+rebuilding+of+schools+in+Pakistan&hl=en&ct=clnk&cd=10&client=firefox?
> )

Plan UK
<http://www.plan-uk.org/wherewework/asia/pakistan/> 

Learning for Life
<http://www.learningforlifeuk.org/> (this organisation was featured in
BBC articles about rebuilding schools in Pakistan)

Care International
<http://www.careinternational.org.uk/Opening++temporary+schools+for+children+in+the+earthquake+zone+in+Pakistan+5904.twl>

I hope this answers your question. Please contact me if you need any
clarification on this answer before you rate it. Good luck with your
initiative!