By default groups are restricted so that only users within your school can send emails to them. This protects your users from unwelcome spam messages.
To edit a group so that external users can send emails ti it follow these instructions.
1 navigate to admin.google.com
2 Use the search box to search for a group. Click the group from the list
3 Edit the group settings by clicking Access Settings link
4 Tick the External > Publish Posts box then hit the Save button at the bottom of the page as screenshot
External users can now send messages to that group