CBSE 10th

Yes, your score in class 10th and 12th are very important parameters if you want to get into any of the IIMs. Though, weightage is given to class 10th and 12th score differ
for each IIM, examples
1) IIM-Ahmedabad - 30% (class 10th, 12th and graduation)
2) IIM-Bangalore - 50% (class 10th, 12th and graduation)
3) IIM-Calcutta - 40% (class 10th and 12th)
4) IIM-Lucknow - 20% (class 12th and graduation)
5) IIM-Kozhikode - 15% ( class 12th and graduation)
6) IIM-Indore - 75% ( class 10th and 12th)
All the above data is tentative. For detailed weightage distribution, check college's official website.
So, by now, I think you have got the idea that each IIM give different weightage to class 10th and 12th. The only common thing in the above data is that each of the old IIMs, as well as new IIMs, give weightage to class 10th &12th. This makes it so important. But the other thing which we should look is almost all IIMs give highest weightage to CAT percentile. So, if you do very good in CAT, you will have a good chance to get in any of the IIMs.
Hope, this answers your question. If you have any other doubts please feel free to write again.