If you don't plan to continue a particular subject in year 12, you
can slack off, but since you chose it, you have the responsibility to attend your classes and study it until the end of the year.
Generally, however, year 11 is not the time for slacking off. You might hear from year 12s who are doing their assessments that they wish they could have jigged more classes in year 11 when they had the chance, but year 11 is probably just as important as year 12.
You need a strong foundation built during year 11 if you don't want to be struggling in year 12. Your year 12 assessments and the HSC are testing what you have learned throughout your 6 years at high school, although specifically targeted at the year 12 syllabus. With subjects such as maths, year 11 topics are included in the trials and the HSC exams, and you need a good understanding of science concepts for year 12 sciences to make sense.
Keep that in mind. Appreciate the fact that your failed year 11 assessments will not count towards your HSC mark, but still do your best.