I agree with buenchico. And you can use OpenOffice which is free and compatible with Word and Excel.
In fact if the homework is being printed out, any word processing software will do.
It should not cost any more for three computers to use your wifi than one or four or five.
If you don't want refurb, look at any of the £300 ones sold by Argos, Comet, Tesco and so on.
This one has 3gb RAM
http://direct.tesco.com/q/R.210-6691.aspx
I would prefer one with a minimum of 2gb RAM and Windows 7 to Vista.