I suspect he's not talking about hiding/masking the sprockets... but rather how to identify if the exposure is correct. The frame number is typically printed on the film in the sprocket area. So if you mask that, then it'd be more difficult to find the correct frame on your negative strips.
I think what the instructor is trying to convey is the concept of finding a proper "black".
The "holes" punched in the edges of the sprocket are obviously not blocking any light at all. The film material itself does block *some* light. The film itself has it's own "dynamic range". The proverbial photo of the black cat at the bottom of the coal mine with no lights turned on should be absolutely black. That "frame" would appear clear on a negative. The opposite photo... the white polar bear in the snowstorm with really really bright lights should be absolutely white -- which appears as a black frame on the negative.
The instructors technique is trying to help make your true pure blacks completely black. Since even a "clear" piece of film technically blocks some light, it would appear to be a lighter gray next to piece of print which didn't have any negative on it at all (e.g. the holes in the sprocket.) He doesn't want stuff which is supposed to _completely_ black to appear to be a dark gray. He's using this technique to find a true "black" point. He wants black to be black.
BUT... this ignores the dynamic range or contrast of the film, paper, and developing process. By making your "blacks" correct, you may be messing up your mid-tones and your whites. And, as you've discovered, all your images appear to be too dark -- which seems to confirm that. You could develop onto higher contrast papers rather than continuous tone papers to boost contrast. Kodak has a tech document loaded with information -- of course it only deals with their own films, chemicals, and papers, but it may give you an idea the types of options available. See:
http://www.kodak.com/global/en/professional/support/techPubs/o3/o3.pdf
Normally you do test strips, then check your images ... paying attention to your highlights and shadows to find the best exposure times and that's what you'd use to print.
As Mully says... you do need to pass. So if you have to make an instructor happy... of course, if the instructor's technique is valid, he or she should be able to demonstrate this technique with repeatable results. If you aren't able to repeat the results using your film, ask the instructor to demonstrate.