Wow, this requirement goes way back to the 1970s.
In the very early days of pre-standardised C, if you wanted to invoke the preprocessor, then you had to write a # as the first thing in the first line of a source file. Writing only a # at the top of the file affords flexibility in the placement of the other preprocessor directives.
From an original C draft by the great Dennis Ritchie himself:
12. Compiler control lines
[…] In order to cause [the] preprocessor to be invoked, it is necessary that the very
first line of the program begin with #. Since null lines are ignored by the preprocessor, this line need contain no other
information.
That document makes for great reading (and allowed me to jump on this question like a mad cat).
I suspect it’s the lecturer simply being sentimental – it hasn’t been required certainly since ANSI C.