Hey Tanya.

You are eligible to appear for CLAT in the same year that you are appearing for Class 12th boards. The eligibility criteria for CLAT says that you need to pass class 12th board exams with a minimum aggregate of atleast 45% marks. As you're still in 12th and your result won't be out till after the exam you are still eligible. Your admission through CLAT would be provisional based on you clearing the laid out eligibility criteria as soon as your result is out. So for now you can choose the appearing for 12th option and don't need to upload the mark sheet as of now.

Best of luck