You can review the subscription(s) in your Inventory/Private Store in the Minecraft Store for Education. If it shows you have “Unlimited Licenses” then that indicates you have a “Free/Trial” subscription(s) that is included by default for educational use. You may download the software an unlimited number of times within your education tenant.
Check out the Try Minecraft: Education Edition for Free article to learn how to take advantage of your free trials, or to try a demo lesson.
If you've already purchased licenses for Minecraft: Education Edition and users are getting a trial message, ensure that you have assigned the purchased licenses to your users, not the trial licenses. To do so, go to the inventory page here in the Store for Education and manage the licenses that show the number you purchased, and avoid the Minecraft: Education Edition app that shows "Unlimited" licenses.