taken from The Princeton Review
You're probably aware that your high school transcript is a record of every grade you earned in high school - and that all of those grades are averaged to produce a single number, your cumulative GPA (grade point average). Admissions officers have their eyes on more than the bottom line. There are other things they examine to get a sense of what kind of college student you'll be.
Freshman Grades - In case you were thinking they weren't important, now you know otherwise. Your freshman grades make up (approximately) one-quarter of your cumulative high school GPA. If you tank freshman year, it will follow you for the next three years. This includes art class, gym, music class, or anything for which you recieve a grade.
Trends - Colleges expect your grades to be consistent or improve. If you received A's in sophomore year and C's in senior year, they will conclude that you are a capable student who became lazy.
Coursework - High school graduation requirements vary from school to school. Most students have the opportunity to take more classes or credits then are required to graduate. If you take only the minimum, colleges will conclude that you are doing the least you must do to get by. If you take extra classes or credits, but they are "soft-ball" classes, this will look just as bad. The best high school transcript will show consistent grades in a challenging course load with challenging classes. Advanced Placement or advanced-level classes impress colleges and tell them that you are ready for college-level work.