If you're using email in a school, one thing you should consider is blocking outside email messages sent to your students. If you're in the United States, then COPPA applies to any students under 13 years old. For most areas, this means all elementary and middle/junior high school students. Some may think that this should apply to all students. That is a decision for your district leadership team.
This goal is very achievable if you use G Suite.
First, arrange students into OUs by school, grade, and/or year of graduation. Personally, I recommend a nested approach. I place student accounts into an OU for their year of graduation. This is easily changed for the small number of students who are retained each year. Then I place these OUs into OUs for their grade. This means that I can quickly move all students to their new grade. Any grade-level configurations go onto the grade's OU, not the OU for the class-of-####. This reduces the effort when students are promoted to the next grade each year. If your district only has one school for each grade (i.e. only one elementary school, one middle school, and one high school), then you can nest the grades' OUs inside OUs for each school, too. This allows a quick way to apply settings across all grades in a school.
If you don't have students cleanly arranged into OUs yet, you may want to consider using either GAM or Gopher for Users to do this efficiently. When coupled with exports from G Suite and your student information system, these can be very effective tools. I recommend GAM for those with no budget and/or lots of experience with the Linux command line and Gopher for Users for anyone more comfortable with a spreadsheet environment.
Now that you have the ability to "aim" settings at the relevant groupings of student accounts, login to http://admin.google.com and go to Apps, then G Suite, then Gmail, then Advanced Settings. Select the OU to restrict on the left side. Scroll down to "Restrict delivery" under the "Compliance" header.
Hover the pointer over that line and the "Edit" button will appear on the far right. Click on that. New settings will appear. In this space, create a list of whitelisted domains. I called mine "Walled Garden". This list should start small and may have a few things added over time. Add your own domain here, as a precaution. Some websites used with students may require registering for accounts over email. You'll have to add those, too.
This may be obvious, but never add "gmail.com" or "yahoo.com" or other free email services to this list. If you do, it will defeat the purpose of this restriction. That said, I did end up adding "google.com" (not "gmail.com") so that students could receive notices of shared files from Google Drive.
You'll also want to add a rejection notice for email that isn't delivered. This goes in step #2 in the above screenshot. You should also check the box to allow bypassing this restriction for internal messages. Note that this applies for Gmail-to-Gmail messages, but you may have external products that technically aren't "internal," such as copiers that scan-and-email documents. This is why your domain should be in the list in step #1. When done, save your new settings. Then duplicate them for any other OUs that should have them. For easier management, I recommend re-using the same whitelist in each OU. For example, you could apply the settings to "Elementary School" and "Middle School", but use the same "Wall Garden" whitelist for each of them.
These settings now apply to both incoming and outgoing email which involve domains not on your whitelist. Note that external users (e.g. "firstname.lastname@example.org") would receive the customized message from step #2 while internal users (i.e. your users) sending out would simply receive an "undeliverable" notice.