In Admin > Comms on the left at the bottom you will see dynamic groups, click this to expand:
You will then see the three new dynamic groups for Covid-19. Click the group to view the students.
You can then click individual students to add them to the contact list to send out comms to or use the 'Add all from' option to add all the students. They will then show in the contact list ready for you to send an email or sms to.