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