My first adventure was run by my oldest brother, when I was about 5 years old, with another brother and his friend also playing. I still remember it.
It was "The Secret of Salt Marsh" from 1st Edition (AD&D). We rolled characters, then played in the same session.
We got to the Carrion Crawler fight at the start and never got any further because my brothers started fighting because my oldest brother gave my other brother a derogatory nickname for his character and kept referring to him that way.
I built a one-shot dungeon for my son's first time playing D&D (used Basic rules).
I converted a Zelda dungeon because he had had a blast going through it. The BBEG was a dragon skeleton and its breath weapon was a string of knuckle bones. We both found out that stacking magic weapons and magic armor makes you VERY tough for Normals to fight.
He enjoyed the conversion and 'borrowed' my 4e rulebooks to create his own character.
I'll only say that for an adventure for nothing but level 1 PCs to seriously consider having CR 0 creatures as opponents and a "boss monster" or CR 1/2 or 1 as the final monster.
Like a jackalwere or two and a pack of jackals. The magic-users can take on the jackals while the martials take on the jackals. And maybe those without magic can light a flask of oil on fire or use a torch or something to damage the jackalweres. There's an AL adventure for lvl 1 PCs that does that and it's pretty balanced. Starts off with the two jackalweres, adds 3 jackals at the end of round 2 and three more jackals at the end of round three.
Well that first adventure took an unexpected turn!
The dice heavily favored the enemies, and three out of four characters were taken down. The cleric managed to drag the warlock to safety, but the enemies captured the other two characters. I was worried it would be a downer for the group, but they're super pumped to come up with a strategy to rescue the other characters next session!
Sounds awesome, there's nothing like the energy and excitement of laying out a game world for your players and watching things unfold. You mentioned you're a teacher (so am I) and I think teachers (and coaches) tend to make good DMs because just as in the classroom, you're trying to keep people interested and invested, to use and challenge their abilities, you know the value of preparation and organization, and you have a good grasp of the progression of a game and campaign.
My first game was with my buddy Gary and his older brother John. Encountered a party of satyrs...I misunderstood the term 'party' and though it was a celebration (of course, I know now that with satyrs, it could mean both).
Draft a simple "jail" that happens to be near to a temple.
Before the rescuing PCs arrive, the captured PCs get to listen in on the conversation as a Priest lets the Guard know that the traditional gods will be more than happy to be given a taste of 'the nectar of the gods' (blood, but he won't say it outright) again. The Priest has some mundane considerations he can exchange - GP, an artwork carving, very fancy dinner suitable for nobles, or a nice non-scratchy undergarment.
This will give them a sense of being under a time limit, even if they really aren't.