Kai is right.
(Frankly, most teachers I've dealt with don't understand the implications of weighting grades or even how to do it properly. I'd say that Sergio in the article is ahead of the game there.)
The question isn't really about how much the final exam is weighted. It's about what is going on in the class to produce students who are getting a B over the course of the marking period and then doing much more poorly on the final. You want whatever grading system you use to reflect demonstrated achievement, so the final grade in the class shouldn't be that different from the average of what the student got on the exams.
FWIW, though--
Actually, having the final be worth 25% of the grade is higher than I've seen for high school courses.
My DD has had some courses that have been weighted more heavily than even that on a final examination or some cumulative project.
It's also not that uncommon in post-secondary settings to find faculty who will offer students either a semester-long, all-graded assessment grade... OR... the grade that they earn on a cumulative (often brutally thorough) final examination.
Very few students do BETTER on a final like that, in my own experience-- but my DD is frequently one of them. If she'd had that kind of deal in place, she'd have straight A+'s in her math classes, and she does not.
As for acceleration causing this, that is just poppycock. INAPPROPRIATE
placement certainly causes it, particularly when coupled with its sidekick, poor instruction. That's quite a dynamic duo for students to work around.
