Meetings will usually feature a lobby, through which the organiser must screen participants and allow them in to make sure they are the expected attendees and accessing the system via Astrea Office 365 accounts. You may want to disable the lobby for sessions with no pupil attendees such as training events, to do this follow the instructions on this link and allow people within the organisation to bypass the Lobby.
The quickest way is to go into the Teams meeting in your calendar, click meeting options and change ‘who can bypass lobby’ to ‘everyone in my organisation’. If you would like to change it back, or pupils will be invited to the meeting, change the value back to ‘only me’