Fix error in Students#Index.
Fix error in Students#Index.

Closes #405.

zqian/gradecraft-development,UM-USElab/gradecraft-development,mkoon/gradecraft-development,peterkshultz/gradecraft-development,zqian/gradecraft-development,UM-USElab/gradecraft-development,UM-USElab/gradecraft-development,zqian/gradecraft-development,peterkshultz/gradecraft-development,UM-USElab/gradecraft-development,mkoon/gradecraft-development,peterkshultz/gradecraft-development,mkoon/gradecraft-development