If I were you I would get LEX/Flex and Yacc and code the Interpreter in that, doing it all by hand seems to be a complete pain in the ass. You may also want to learn GCC GAS so you can edit your C/C++ at the Assembly level. Making a very basic, BASIC interpreter isn't to hard if you are using a Lexical Analyzer (LEX/FLEX) and a Parser (YACC). If you use the standard C Library to make a language it is all basically writing the Syntax, the rest is pretty easy, and if you want an A+ I would include a Graphical Library and maybe a Networking Library. Personally I own
http://www.amazon.com/lex-yacc-Doug-Brown/dp/1565920007 however; there are some pretty good tutorials on the Internet that would be a good start, remember, for a school project it doesn't have to be an entire language, maybe just a program that can understand user input, maybe analyze a sentence and respond accordingly, e.x. This is a question, eat is a verb, food is a thing that is good. Will you eat food? Computer responds; Yes.