The documents sent by the teacher in the WeChat group should be signed by parents. You can take the following steps:

1. Open WeChat and click the Settings button on the "Me" page.

2. Click the gear icon in the upper right corner.

3. Click on the group chat name to modify it.

4. In the column of "Remarks", put a tick before "Parents please sign" and click the Finish button.

I hope the above information can help you solve the problem. Please feel free to let me know if you have any other questions.