To reiterate points made in other answers and comments: no, in terms of learning things (!!!), it is absolutely fine to make use of other peoples' prior work. It would be laughable if we all had to reinvent the wheel, etc.
Yet, yes, there are some forms of academic stuff wherein there are "rules" prohibiting looking at all the stuff out in the world. The most ridiculous type is "it's not ok to use an idea not covered yet in the course".
The latter concept only makes sense if "education" is an exercise in conformity to authority, rather than ... education. It's ridiculous.
Nevertheless, I hear gossip that some people do ridiculous things. Incredible... in a bad way.
EDIT: yes, certainly, as mentioned in comments (and as many people know), there is substantial reason to understand "what implies what", and often a sort of annihilating over-kill is far less enlightening than a more-restricted-means explanation.
In fact, questions which may be extremely awkward from a too-elementary viewpoint that become transparent from a more sophisticated viewpoint are things that I myself like to emphasize to my students in graduate courses. Not everything does yield to a more sophisticated viewpoint, of course. But quite a few of the introduction-to-advanced-math questions are indeed hardly tractable from an elementary viewpoint (and this is visible historically, motivating a great deal of modern math!), but/and become mundane from our contemporary viewpoint (which was motivated by wanting to mundane-ize such questions, hm!).