Seems it does work. I used testingcheatsenabled true and aged her down in Create A Sim. Now it still says she has a degree from LeFromage, but in the school/work tab it says something like, "There are no schools in town to go to."
When I click on the school it says I have an option to "Start High School" (and the regular options), and I also got a notification of the prom coming up, even though my character has already been. Slept until the next day (which was Fri) and she stayed home, so that clears that up.
I let my parent sim go on a vacation when I last saved, so I don't know if she can be enrolled in another Boarding School. I don't remember if a teen sim can enroll themselves, It's not in her own options or her computer, so I assume a sim can only attend one.
