Also, I have already taken the dev-tools course and it was very useful.
Depending on how much time you sink into learning, you can progress at a very high rate. You want people to make the most informed decision possible when hiring you, so you want to be showing them the thing most likely to wow them.
Keep in mind that some companies with an IT Department may still outsource smaller projects to freelancers. In this case, your development practices may be evaluated by IT minded folks. You really do not want to lose out at a job because you are showing code 4 years old, and that's not even how you'd go about it if you were to start over.
Another poster did say something about traffic increase. But let's extend that. If you can prove that you not only redid the design, but you increased performance by X%, and traffic by Y% and so on, those are real numbers people who make decisions can base you on. So it may be worth your while to learn about metrics.
In School, I would really look into a course on statistics. You'd be surprised how often that comes in handy.
At least in the Case Study homepage, there is a lot of commented out markup in the source, and an entire section where it looks like it was done in an HTML generator - because you are not using the Cascade at all, and every link re declares the same font-family over and over.
Something else you should consider is looking into this course: http://discover-devtools.codeschool.com/
It will help you understand what is going on when debugging webpages.
I know it's hard to be doing stuff that likely few people your age can relate to. Adults will dismiss you because of your age, and there isn't much you can do about it aside from continue to develop your skills so when you are taken seriously, they feel stupid for doubting you.