Kerry, you might want to check closely the current interpretation of the class size restrictions. One of the things that I came across recently was that they can't double count students that might be scheduled for two classes at the same time - the specific example on the FLDOE FAQ is for students enrolled in both ESE and mainstream classes in the same session. Maybe you could get around the class size restriction by counting the sixth grade class as an ESE accomodation in her EP while she is counted for class size purpose in her regular grade LA class. Even though it's a core class for the 6th graders, it could be considered an enrichment class for your DD.
This would only work if the school has space in any other class to actually count her there, instead of in the 6th grade class.
One other option - could she 'audit' the class, for lack of a better term? She goes to the class, but is not officially registered and you treat it as a homeschool class, that she just happens to get in that classroom?
Good luck! Hope you get something figured out!
