Student counter isn't updating for teacher's attendance view

[ThatXliner]

Prerequisites

• [X] I have searched the issues and believe that it has not already been reported
• [X] I have made sure this bug reproduces on the latest version
• [X] I agree to follow the Code of Conduct

Bug description

See title. Basically, when a new student uses an attendance code ("joining" the attendance session), the student counter on the teacher's side doesn't get updated

Reproduction steps

Self explanatory

Other information

No response

Reproduces how often

I can reproduce this bug 100% of the time

[ThatXliner]

• Blocked by #26 because this issue implies refactoring