If there aren't any national parks near you, you can still explore them through some nice online resources. National Parks virtual tours are available in the Google Arts & Culture apps for Android and iOS. If you have VR headsets available to you, take a look at Google Expeditions virtual tours of the "hidden treasures" of National Parks.
Over the years PBS has produced many videos about the National Parks. You can view some of those videos in their entirety on the PBS video website. Search on the site for "national parks" and you'll have a big list of videos to view. Here's a list to get you started.
Web Rangers offers seven categories of games about different subjects related to the National Parks. The game categories are people, animals, parks, science, history, nature, and puzzles. Each category contains games of varying difficulty rated from easy to difficult. Some of the game topics include dendrochronology, animal tracking, animal identification, fire fighting, and map reading. Students can play Web Rangers games as visitors or as registered users. Registered users can track their progress and earn virtual rewards. Registered users can also create their own customized virtual ranger stations
The National Parks Service's Digital Image Archive is an excellent place to find images of U.S. National Parks. You can search the archive by park and or subject. All of the images are free to download as they are in the public domain. The National Parks Service also offers a b-roll video gallery. The videos in the galleries are in the public domain. The b-roll video gallery can be searched by park, monument, building, or person. All of the videos can be downloaded. Some files are quite large so keep that in mind if your school has bandwidth limits and you have all of your students searching for videos at the same time.
Google Earth offers a great way for students to view national parks in the United States and beyond. Your students can explore imagery in Google Earth to learn about the topography of a national park. In a lot of cases there is Street View imagery available within national parks and UNESCO World Heritage Sites. Your students might also benefit from viewing tours within Google Earth.To locate a tour you can refine a Google search by file type to .KMZ and then launch the tours that appear in your search results.