I made a Discord bot recently, just to see how it works. Troll 9000 is pretty simple, it just reacts to any of my messages with a 👑, and my friends’ with a 💩. (If you want to check out this masterpiece, the source code is available here.) To make your own and add it to a server, follow the instructions in this Discord.js Guide.
You can set up a Node.js server in replit, even from a school computer. (Replit can talk to the Discord API, so if your bot is running there you can test it in real time.) Start with this Example Project.