Well written and fairly effective, but I agree with Nightfish - you need to have some critical assessment in there. It's a paper, your teacher probably isn't all that interested in 'This game is awesome'. You need to show a bit more analysis - why is it awesome, and what aspects might possibly detract from it's awesomeness?
Even if you personally can't find fault with anything in the game, try to look at it objectively, read as many reviews as you can, try and find something with even a vaguely negative connotation that you can balance against the positives. You've got the length (or lack of) in there, keep going. You don't discuss graphics or sound (the age of the Source Engine, versus it's ability to play on a wide range of hardware) or even mention the level design much, or the ever present black humour.
Going back and reading it again, not much more than half the essay is actually about Portal. And for someone who has no concept of what Portal is, would they be any the wiser after reading your review? Maybe clarify what portal is, and the objective of the game. You don't really make it clear that you walk into one portal and come out of the other, which could be somewhere else entirely. Obviously we all know that's how it works, but your teacher may not.
Sorry if this comes across as overly negative, it was not my intention. Basically focus more on the game - what it is, how it works, and how the pros and cons balance up.
EDIT: It might be worth mentioning that the pc version at least can use custom levels, as this impacts on replayability and user interactivity.
