It comes down to how the school does it. Extension work is allowed and that may include 11 and 12 work. They cannot have a topic that is a year 11 or 12 topic.
All schools get involved in this, it is just a question of how they enhance their 9 and 10 program.Schools can choose to enhance their Year 10 program to prepare students for their Preliminary courses. This is different from commencing the Stage 6 program during Year 10. Schools would need to clearly differentiate between any enhancement to Year 10 programs and commencement of the Preliminary courses before day 1, Term 1 of the Year 11 calendar year.