Should you justify or left-align text when writing? I've written a 5,000 word essay on a topic of my choosing.

While reading through my writing, my teacher suggested I justify my text, as opposed to left-aligning it, as this looks neater.

I agree, finding it looks much better; however, after looking around, many people stated that the justification of text makes it much harder to read. Also, most of the sources I consulted, as part of research, used left-aligned text.

Is it just a subjective matter, or is there a standard?
Look for the specification and implement it.

In your particular case, your teacher specified that she prefers justifying, so you should act accordingly, since she is the one who will grade your essay.

Later in your academic career, you may come into situations where the format is specified by an institution rather than an individual person, for example, a journal or a funding agency.