Please check your Microsoft Teams name and security settings.
To do this, click on the three dots to the right of the Team name in the list and select Edit Team.
Please name your team beginning with your school initials, followed by the teacher's last name and then the subject. Staff teams don't need a person's last name but should start with the school initials.
Then be sure the "Privacy" setting is set to Private - Only team owners can add members.