While filing i485 it is asked to include birth certificate or could be replaced by
school certificate if not available. In this context certificate has all dates in Bikram sambat(BS) including DOB.
Would this be still be sufficient document with translations?
Any suggestion?

Why would you try to cut corners?
Your birth certificate should be prepared in a day. Add 3-4 business days for shipping. Total time: ~1 week.

I don't know if School Certificate can be used in lieu of Birth Certificate. I had the same issue as I did not have a birth certificate . I had the birth certificate issued from my ward. I had 2 copies made ( In English and Nepali). Anyway I did not get RFE using the document from the ward office. I guess just make sure its in the office letterhead with visible logo, phone number and address.

I actually got a RFE in regards to using the nepalese version of the birth certificate in my original applicatin. So, I had to get a new one with the nepali and english version of the birth certificate from Kathmandu Metropolitan office. It seems they provide that these days so good for us now. I don't recommend you to submit it without getting the english 'birth registration' document from the Kathamndu or whichever district your's applies to in Nepal.