If you or your students have trouble scrolling, make sure you've selected the arrow key tool.

That's the tool you use to select items on the screen. Selecting it will make sure that you don't have any other tool selected. Then be sure to start scrolling by clicking/tapping on the background, away from any interactive elements.

Finally, be sure there's actually content to be scrolled to. If your students are accustomed to long Scrolling view activities, and you deliver them a nice, compact Slides-view document, they might need some guidance on how to navigate from slide to slide.