First, get set up to easily hop back and forth between the Atlas in your browser and your video calling app. Add a shortcut icon to the Atlas on your Home Screen for quick access: with the Atlas website open, tap the Share button the Share button then tap "Add to Home Screen".
To toggle the Teacher Atlas and your video-calling app, double-tap the home button to show a visual of all the apps you currently have open. When those icons pop up, select the browser you have the Atlas open in. If you have a newer iPad model that does not have a home button, swipe up from the bottom to the middle of your screen and hold until you see the App Switcher. Swipe left or right to find the app that you want to use. Then tap the app.
When using your iPad with Zoom for teaching, the student will lose your video feed when toggling over to another app (the call doesn't disconnect; they just don't see your video anymore). Zoom's most recent updates make it possible to screen share from your iPad, meaning that if you wanted to show them what you're looking at on the Atlas, you can easily do so. This option is in the top-right of the window when using the app on an iPad. There's even a way to share your system sound, which you can do by returning to the Zoom app, double-tapping the middle of the screen, and selecting "Share sound."