It may be that by accident you choose the wrong student, this is not a problem you can always reverse your action.

1. Choose in the search options " My recently completed tasks" (5 days visible in this overview) In the detailed screen of the student, these tasks are always visible. 
2. On the screen you will see the task. 
3. And the date and time it was last edited. 
4. Click on the little dots on the right hand side and choose " Revert  "  (! Do not revert Date Picker tasks)