can't join a team even with the right code

TrungNguyenyou could ask your teacher kindly to reset the 'Join code' for the Team. Perhaps, a new Join code will work better. The teacher can get a new join code for the Team by clicking on the 3 dots beside the Team name>Manage Team>Settings, expand the section for Team Code and click on 'Reset' (see diagram below). This will create a new code that they can copy and send to you.

If the Join code doesn't work, they could also click on the 3 dots beside the Team name>'Get link to team' and send you this link. When you click on the link, it will allow you to make a request to join the Team, which your teacher will then have to approve or decline. The only other thing to make sure is that you are signed in to Teams with an address in your organisation, as Team membership in your Teams environment might have guest access turned off.