Hey, fellow classroom teacher here. While I have not actually done D&D in a school setting I did run and/or supervise it at a summer camp over several summers for similarly aged kids. That was under the aegis of a creative writing project, so we used nothing prepublished except the rules, and I tried to incorporate materials made by the kids (including one summer having the whole adventure take place on a map one kid drew). Most of the time in my experience once the kids get going they require minmum help, and while I have not experimented with prepublished materials I suspect most middle schoolers would not stick to them for long, so really the key is to give them something as a jumping off point.
My personal preference in your situation is, as others have suggested, for
Lost Mine of Phandelver from the original 5e starter set. It's a modern classic if anything is, the module has more DM supports than a typical WotC product, it is a slim handbook with all the monster and magic item stats in the back so the DM only needs MM or DMG if they add something, and it's usually cheap on Amazon. It is a bit of a sandbox for when the group inevitably goes off the rails. I personally DMed it having never played 5e D&D before, and it was fun.
A strong alternative is the new
Heroes of the Borderlands starter set. It provides even more DM support, is even more of a sandbox, and comes with all sorts of bells and whistles like all the maps you need and tokens for all the monsters. The tokens, maps, etc. might be useful for other groups as well. It is also set up so that DMing can be shared out between various people, with three seperate booklets for the Keep, the Wilds, and the Caves of Chaos. The drawbacks are that it is $50, has lots of items to keep track of and try to fit back into a packed box, and that it relies on so many fancy apparati (having cards for all the monsters and npcs, etc., not making you draw your own maps) that I don't feel like it really encourages a DM to improvise beyond the provided materials or to generally develop the skills they need.
In your shoes I would probably decide on the best option based on my best judgement of what was a good fit for the would-be-DM.
If you haven't requested an official D&D for educators kit yet you can do so at
https://dnd-support.wizards.com/hc/en-us/articles/9485614877588-Educator-Resources