Rule #0: Have fun.
Players can chat about what their characters can do and what their characters may want to do in any given situation.
Roll a 20-side die to determine how successful any action is. The Dungeon Master (DM) determines how difficult an action is. The higher the number, the more difficult it is.
If players are attacking, roll a damage dice assigned at character creation. In reality, a simple 6-side dice will get the job done just as well. The higher the number, the more damage done.
You really can make up the rest and incorporate as many or as few traditional D&D rules as you like.
However, I found the following documents and posters to be instructive.
How to be a DM (guide)
D&D Beyond Free Rules (2024) (This is not light reading)
Go play or at least read through Peril in Pinebrook (it's very short).
One round of this, and the rest made so much more sense for me and my students.
Rules-lite, premade characters, and it provides a script on what to say and do.
Apply for the Educator License from D&D Beyondto gain access to the Player's Handbook and the Dungeon Master's Guide. START with the D&D Beyond Educator Resources
On the same page, submit a request for the Educator License.
This grants you access to a plethora of free resources via web browser and the Beyond app.
Six Abilities Poster to print and hang in your classroom.
Difficulties Class Examples Poster to print and hang in your classroom.
Glossary of common terms in D&D
Basic Actions for Players (good one-pager to print for players)
Dungeon Masters Guild for an endless supply of affordable and often free adventures.
Running a D&D Adventure YouTube guide