I am a senior in high school as well. I never took any classes because, well, they were not ever offered.
It's ok though. I've a nice bookshelf ranging from HTML to PHP/MySQL to Linux and just to about anything else you can think of.
When trying to explain to someone what PHP is, most of the time I have a difficult time trying to explain the difference between server side and client side coding.
So basically, just hit that hard, and you should be ok. PHP is a great and easy language to learn. You should do just fine.