Awkward to get light and tough, I'd say.
This morning in Asda Living I saw a Compaq CQ56-1025A 15.6" laptop reduced from 327 to 277. That has a decent size keyboard, though for prevention of drinks/ crumbs getting into laptop (and sensible distance from screen, if used at home on a reasonable size desk) I would suggest a multimedia keyboard (has volume control, mute, jump to next/previous track, for use with Win Media Player [which he might use for MP3s and CDs] etc) which may be found for under a tenner (I should think) on Ebay. Means one uses the full set of keys and if switching to a different machine 2-3 years later, no need to get used to yet another keyboard layout (only really important functions on laptops are related to volume/ brightness and wi-fi controls sometimes through the laptop 'function' (fn) key and various other keys whether normal function or up/down etc). Those vary from laptop to laptop and are hardly needed on a frequent basis so a multimedia keyboard would do 99.9% of what one needs.
You would not get Office/Word included. I think Asda sells that at a further 107 pounds. However, OpenOffice (now probably LibreOffice) is available for free and should be able to save MS Word 2003 compatible documents which could be used at school. Would be worth checking with school as to what they recommend and whether they have any scheme for software licences/ purchase to save you cash, and if they insist on Microsoft it shows they are short-sighted about how it just benefits MS with thousands of pounds spent when other software is capable and cheaper or (like OpenOffice/ LibreOffice) FREE.
(Sorry, have a particular 'bee in my bonnet' about use of MS software as if there's nothing else, and how much is spent in local/ national government on it, where some other regions [eg in Germany] have adopted open source software as a cost saving measure, just as good, with no 3 year life cycle of redundancy, which is what Microsoft builds in, so they have a revolving door source for neverending income!)