Thank you for asking.
Once an activity has been submitted, it cannot be unlocked, so students need to make sure they are done with an activity before submitting. However, you can always allows your students to redo an activity by copying it, pasting it and resending it using the button at the top right corner of the activity, as shown below.
Lastly, we have plans to add a confirmation prompt which will reduce the possibility of students submitting an activity by accident, so stay tuned!
I hope this helps and let me know if you need anything else!
The EF Class Team