Hmmm when I got my ticket 2 years ago traffic school was only an option if it was <25mph. Mine was >25mph so I didn't even get a choice.
You actually have to go to court, see the judge, and request it directly from him. If you choose to plead guilty and just pay the fine through the mail, than yea, you won't have the option to. The key is the court clerks, who handle violations where the defendant pleads guilty and pays without going to court, can't authorize it.
edit:
"
(b) Authority of a court clerk to grant a request to attend traffic violator school
...
(2)
Ineligible offenses
A court clerk is not authorized to grant a request to attend traffic
violator school for a misdemeanor or any of the following infractions:
...
(G)
A speeding violation in which the speed alleged is more than 25 miles over a speed limit as stated in Chapter 7 (commencing with section 22348) of Division 11 of the Vehicle Code; and
...
(c) Judicial discretion
(1)
A judicial officer may in his or her discretion order attendance at a traffic violator school in an individual case as permitted under Vehicle Code section 41501(a) or 42005 or for any other purpose permitted by law. A defendant having a class A, class B, or commercial class C driver's license may request to attend traffic violator school if the defendant was operating a vehicle requiring only a noncommercial class C or class M license. The record of conviction after completion of traffic violator school by a driver who holds a class A, class B, or commercial class C license must not be reported as confidential. A defendant charged with a violation that occurs in a commercial vehicle, as defined in Vehicle Code section 15210(b), is not eligible to attend traffic violator school under Vehicle Code sections 41501 or 42005 in lieu of adjudicating an offense, to receive a confidential conviction, or to avoid violator point counts."
http://www.courts.ca.gov/cms/rules/index.cfm?title=four&linkid=rule4_104