Ok, so I am a bit late in the reply.
To know which dictionary to use, you can check out what the school
has in its library. I'm sure these dictionaries are chosen by the
school for their usefulness, so, it would be useful in your
evaluation.
And, erm, the Oxford English Dictionary has twenty volumes, practically like an
encyclopaedia, so, I don't recommend buying that, unless you wish
to bring an entire book shelf to school. 
Not really important, but you'll be surprise that we're not
entirely British in our usage. Examples include 'can' instead of
'tin', 'Scotch Tape' instead of 'Sellotape', and of course
'encyclopedia' instead of 'encyclopaedia', etc, etc, etc. We also
like to use 'ize' instead of 'ise'. These are American
usage/terms.
WJTW
