Well, first I must admit that it's been more than two years since I went through this process and I have forgotten most of the steps. However, I searched a little on the internet and found following:
Yes you'll have to have IBCC assessment. In fact, IBCC is supposed to send your attested copies directly to WES. So, what you'll have to do is to go to the IBCC office with your documents, already attested by the inter and matric boards, and have those attested by IBCC. Once they're attested, you'll then take the WES form and request IBCC to make a sealed envelop, that'll include your IBCC attested documents and the WES form. That sealed document will either be sent directly by IBCC or you'll have to mail/courier it yourself to WES with IBCC's mailing address written on it.
Following is the detailed process that I found on another thread of this forum:
Do ask away in case of any confusion, I'll try to help as much as possible.