How do I deploy an app only to a subset of iPads at my school?

The best way to do this is to create a Smart Group for each app. Here are the steps.

Create a Smart Group
1. In Workspace ONE, click Groups and Settings (in the menu on the left), choose Groups, and then choose Assignment Groups
2. Click Add Smart Group (at the top)
3. We recommend you include your school's four or five character abbreviation and also the name of the app. For example: GSCA - Duolingo
4. Click Devices or Users and then click OK at the warning box that pops up.
5. In the Devices box, enter the names of an iPad and click Add (unfortunately, there is no easy way to add multiple iPads).
6. Click Save

Deploy the app to the Smart Group
1. Click Apps & Books (in the menu on the left), choose Applications, Native, and then click Purchased
2. Click the name of the app. If this was the first time you are deploying the app, you will need to click the Enable Device Assignment button.
3. Leave a zero in the field for Licenses on Hold, and then click Save & Assign.
4. If you have previously assigned this app to other assignment groups, you will see the Assignment window and you can click on the Assignment Name. If you have never assigned this app to any other assignment groups, you will see the Distribution window where you can enter a name. We recommend you enter the name of the Smart Group as the Assignment name.
5. Click where it says To whom do you want to assign this app? and select the Smart Group you just created
6. In the Allocated field, enter a number that is less than or equal to the total number of licenses you purchased from Apple School Manager, and also is greater than or equal to the number of iPads to which you plan to deploy the app.
7. Leave the radio button for Auto selected.
8. Then click Create. Then click Save. Then click Publish.

In the future, if you want to change which iPads have the app, just add or remove iPads from the Smart Group.