The big question is, "Why do you want to learn Greek?" If you want to be able to carry on a conversation with modern Greeks, you need to look elsewhere. If it's to be able to work with a Greek Interlinear, a concordance, and maybe even a Greek New Testament, then Basic Greek in 30 Minutes a Day is a great place to start. In a short time, you can learn to recognize the letters and pronounce the words or at least read them. This will give you enough to be able to do some work in reference resources and do a more thorough job with Bible study.
This book makes the job even easier by starting with quite a few cognates (words that sound very similar to familiar English words). Simple written exercises accompany brief explanations to reinforce learning.
The book is really designed for self-study. The answers are all in the back of the book for self-checking. A high school student should be able to work through this independently with only occasional assistance. At the end of the course, a student is likely to be able to read a Greek New Testament (probably very slowly!) and know how to look up unknown vocabulary words and figure out the meaning of words for himself. He is not likely to be able to write and converse in Greek.
It will probably take far less than a year to complete this book if a student works on it everyday for a half hour to an hour.