Can't Unlock Student's Test

Tests can be locked/unlocked at any time on the Test Monitoring page. If a user is unable to lock/unlock a student's test, it would be because the test library's permission are limited to a higher user level than the current user is set to. A test library's permissions are set when the library is created (See: Library Builder, specifically Step 4 - Users, for more information.)

After a library is created, it can be edited and the user level can be changed. (See: Library Builder, specifically Edit a Library, for more information.)