I have compiled the following from Microsoft Help articles.
"If chat is enabled in your class team, you can ask your teacher a question privately using that feature. Select the Chat icon to open a new chat and type your question there.
If you can’t open a new chat, your school doesn’t have that feature turned on. Use @mention with your teacher’s name in a channel to ask your question instead. Other class team students will be able to see your question, too." Click here for the full article.
"When you're in a meeting, select Show participants in your meeting controls to see the people who were invited but haven't joined yet.
To ask someone who was invited to join the meeting, select their name > More options > Ask to join .
You can also type a person's name or phone number in the search box to add someone who was not previously invited." Click here for the full article.