SPOILER ALERT: This blog post includes actual math.
So, here's the thing: the grade point average is a simple mathematical average. You know, add up all the points and then divide by the number of values you added. An average gives a snapshot of the list of values as a whole--once you even them out, what's the take home message? The important part is that the averages evens out your performance. This is good for you if you have one really low grade; it's unlikely to bring down your GPA substantially. But it also means that one or two or even one semester of spectacular grades is not going to budge the GPA very much higher.
Let's look at an example. In the standard 4.0 grading scale, your grades are each assigned a point value, something like this. This is a simple, unweighted grading scale. We'll get to weighted grades in a minute. Let's say you've been getting A's and B's pretty much until the second semester of sophomore year, when you get a C in Phys. Ed. because the coach doesn't like you or whatever. Assuming you're taking six college-prep classes per semester (English, math, science, social studies, foreign language and some elective), you've earned 59-ish grade points--3.3 points average for each class. Then second semester sophomore year, you add 18.5 grade points instead of your usual 19.8. What does this do to your average? Well, 59ish + 18.5 = 77.9 / 24 (total number of grades you've earned in high school so far) and your GPA is now 3.25, down 5 hundredths of a point from your 3.3 average. You would still round this up to a 3.3, so you took very little hit from that C in gym. Yay for averages!
Now assume that you've finished your junior year with that 3.3 and you decide you want to apply to the Ivy League, so you need a 4.0. All you have to do is do fantastically in those classes senior year, right? Not so fast. 3.3 x 36 classes = 119 grade points, approximately. If you take six classes in that first semester of senior year and ace them all, you add 24 grade points for a total of 143. But then you have to divide them by 42, the number of classes you've taken up to that point. And you're sitting pretty with a (drumroll, please) 3.4.
Boo for averages!
If 4.0 means straight A's, how do people graduate with more than a 4.0?
They are using a weighted average, which means the more difficult classes are given a bonus point. The idea behind weighted averages is to give credit for the amount of work involved in earning the grade. A student who earns an A by reading 100 pages of Moby Dick a night for two weeks and then writing a 10 page critical analysis should earn more points than a student who took a month to read The Fault in Our Stars and wrote a one paragraph book report, right? So weighted grades assume that a B in an AP or college-level class is equal to an A in a regular class. Students who take all or mostly honors, AP, IB or dual-enrollment college classes will end up with GPA's higher than 4.0, even if they don't get straight A's in those classes.
So, let's go back to our "raising my grades at the last minute" example
If your 3.3 GPA has been earned in regular classes, and you suddenly switch to all AP classes and ace those, your GPA would become 119 + 30 = 149/42 classes = 3.5. In order to raise a 3.3 GPA to a 4.0 in one semester, you'd have to get 7.0 grade points in each class. I just don't know a school where that would be possible.
The Bottom Line?
If you want a high GPA as a senior, you need to start freshman year. On average.
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