Any class when they are ready

SAT stands for Scholatic Aptitude Test. SAT exam is conducted by the College Board and is required to be taken by students seeking admission in undergraduate schools. 

So the eligibility for the test would be completed +2 or about to finish +2.

Applicants looking for the undergraduate courses, particularly in US and Canada, are required to take the SAT exam.

Exam would comprise of the following sections:-Math, Reading, and Writing and there is an optional section which is the Essay part, which has one 50 minutes long substantial passage from which the student needs to build their argument.

The total duration of the test would be 3 hours with the essay portion added.

The test can be taken around 5 times an year.