The Answer Sheets tab becomes active after the initial save of the Classes record. Each student in the class who took a test will have a corresponding Answer Sheets record after the Scantron wizard is run or the answer sheets are manually entered to the Classes record.
You can add an Answer Sheet record manually to the class record from the Answer Sheet tab. Generally this is done for non-scantron type tests such as essay or short answer tests where the answers are more subjective. The record will only be able to indicate whether the student passed or failed because the scoring fields are only populated through the Scantron wizard process.
See Creating an Answer Sheet for more information about manually creating the Answer Sheet record and Using the Scantron Import Wizard for information on automatically creating Answer Sheet records with the Scantron Wizard.