Beside the value of CAT scorecard your marks starting from class 10th and all the extra and co-circular activities you have gone through matters as the management of yours and how much dedicated to studies as well as your versatility is what taken into consideration.
Yes, 10th and 12th marks are
necessary for getting shortlisted in
IIMs. All IIMs and some other
institutes have their own process of
shortlisting students where they take
into consideration the 10th%, 12th%,
graduation%, workex and other
details.