Spelling City  allows Teachers to easily create spelling tests for students, this is free of charge; upgrades are possible but the basic free service is excellent. It is now even more useful as teachers can add their own sentences to define /describe a word. Students could create their own tests, all free and no registration required.

For example – can you spell words associated with triangles and quadrilaterals?

The UK GCSE specifications for teaching from September 2010 include marks for quality of written communication.
Some questions will include marks for this; to quote AQA:

‘Candidates will be expected to use correct spelling, punctuation and grammar in any explanations they are asked to provide.’