why .tar? That's mostly used in Linux/Unix? Are you working on Linux servers or something? .zip is a really common format, and most any Linux server can compress/decompress it... If I were you, I'd maybe ask the teacher what the deal is behind this...
Anyway, in winrar, when creating a new archive, it only seems to allow .rar or .zip. I know it'll decompress .tar, but maybe it can't create them?
EDIT: tar really isn't a compression, more of a package. It puts the files together and keeps the file permissions and ownership in tact. Usually linux source files are .tar.gz, which means the gzipped the tarball package to compress it, because the tarball really isn't compressed/shrunk at all.