1. Tee-shirt design and sales. There are plenty of online services that let students design and sell tee-shirts without any start-up costs. SunFrog is a service that I have personally used for that purpose.
2. Drone piloting. Students who have drones might offer their skills for sale to real estate agents. I know one realtor in my area who has hired students to fly drones to photograph the properties they are listing for sale.
3. YouTube publishing. YouTube allows you to monetize your videos through the use of their ad network, AdSense. Students could publish tutorial videos for their favorite games, demonstrate DIY projects, or publish videos about any other topic that strikes their fancy. It takes a lot of video views to make significant money this way, but it's not unrealistic for a teenager to make $50-100/month.
4. Design and sell 3D printed objects. I've seen students create cell phone cases and speakers with 3D printers. A simple e-junkie or eBay store is a fine platform for resale of those items.
5. Virtual tech help. This has been an in-person option for years, but free tools like Skype, Zoom, and Google+ Hangouts make it possible for students to offer tech help remotely.
Disclaimer: Most online stores and advertising programs require people to be 18 or older. Therefore, students will need to have their parents register and let their teens manage the materials sold. Depending upon how much students earn, there may be tax implications to consider.
