What will happen to the students who haven't uploaded 12th certificate on jee advanced portal 2018? (As of now my board results are not announced)

When you report to an IIT campus for getting admission, they will ask original documents like 12th mark-sheet, caste certificates to check the authenticity of whatever you claimed and uploaded. So whether you uploaded 12th marksheet or not, they are anyways going to check it then.

As for caste certificates, it is a must to upload them today because for those students a separate list will be made.