Grade 6 - I think your teacher isn't looking for cut-and-paste off the internet jobs - he or she knows what your level of musical education is, and what to expect from you.
If you want to go on with music, it would be worth getting a good musical dictionary. We have Slonimsky and Baker's "Biographical Dictionary of Musicians", because it does have reasonable coverage of 20th century musicians. A new edition came out last year, which can only mean that the edition we bought is now available cheaply second-hand.
Although the dictionary is "biographical", there is plenty to get you started on thinking about each composer' music.
If you google with the composer's name plus one word that you would expect to find, e.g. "Tchaikovsky" + "orchestration", you will certainly get several paid subsciption sites that are not accessible to us ordinary folk, but you will also get things like programme notes from an orchestra, Wikipedia, and the like.
For example, one of those sites talks about "lush orchestration". Lush means luxurious or more than adequate - so next you want to see if it refers to the notes (harmony, melody) or the sound (instrumentation, dynamics). So keep googling...
Also, when you find terms like this, why don't you look at your own music, or go to a big library and get out a score or borrow some CDs (or listen on you tube, and try and figure out what you think that term means. I'm sure your teacher would rather hear that you were looking and listening for yourself at Grade 6 level.
When my son took Grade 5, he went over his music and identified common chords and changes of key - even doing that showed him which composers changed keys most often, which composers used lots of notes apart from the basic ntoes of each scale, which composers stuck to the basics for harmony, etc. etc. Since you're doing Grade 6, you can put your Grade 5 Theory to work!
I don't know the first thing about music, so if I can get from A to B with a dictionary and the internet, I'm sure you can do a really good job.