Only the admin of the group (generally a teacher) can cancel the one-way communication in the chat.

First click on "Chats" in the footer.

You can then select the relevant group chat or individual chat in the overview of the chats suggested to you.

Suggestion: You can also use the search bar to enter the name of the chat / person to find the relevant chat more quickly.

Here click on the gray tab "OneWay" in the chat.

In the following window you can either open the chat permanently or set a time limit for opening the conversation.

If you have only opened the chat for a certain period of time , the counter will show up until when the conversation remains open. 

If you have opened the chat permanently and would rather close it now, click on "Click here for group details" in the header bar of the chat window.

Then select the "Settings" tab to call up the following menu.

Clicking on the "One-Way Settings" tile opens the following page. All you have to do here is select "Activate One-Way".

Now the chat is closed again so that only the group admin can send messages.