I cannot get my student(s) logged into a testing session.

Make sure the student is on the active roster assigned to the event. See Manage Rosters, and step 6 of Create a test event for more information.

Check that the student you are encountering issues with is assigned to the correct location and grade. See About managing students for more information.
Check that the students location is included in the events Participant List. See Create a test Event Step 5 for more information.
Ensure the test session created is for the proper grade and level of the student(s) you are trying to test.
Finally you should confirm that the test event has an assignment that covers the students' grade and location. See About test assignments for more information.

If you are unable to test a student in any current test event setting, there are a number of steps you can take to troubleshoot the problem.