TCC - just like many other C compilers - supports many GNU extensions.

The professor could have just insisted on `-std=c99` or a similar GCC flag which disallows GNU extensions.

When I taught programming (I started teaching 22 years ago), the course was still having students either use GCC with their university shell accounts, or if they were Windows people, they would use Borland C++ we could provide under some kind of fair use arrangement IIANM, and that worked within a command shell on Windows.