My absolute all-time favorite is magicdraw UML. There is a free edition which is probably too limited for the assignment, but you can also get demo licenses that work for a few months. Perhaps they can give you a longer tryout period if you say you are using it for your SCEA assignment (suggesting of course that you would recommend it to your boss to use at work).
SCJP, SCJD, SCWCD, SCBCD
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