I never bought one and I never felt like I was missing anything. Your school library will undoubtedly have a giant Black's LD, and dictionary.law.com is useful for the less obscure stuff.
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There is not necessarily any relationship between being an "A" student and being talented at teaching.
I would recommend to the OP to put the actual class rank, since you know it and it seems less strange than "top 6%" and more favorable than "top 10%". So something like: Class Rank 12/216 or 12 of 216. Congrats!