I would try some vinegar and baking soda, put some on a sponge or dish rag, run the microwave about 1 minute at a time. The steam created should loosen the burned on food, the baking soda should absorb the smell. Then you can use the rag to clean it up with, just be careful, the rag will be hot, just how hot depends on the wattage of your microwave, and how many "1 minutes" you have run.

If that fails you could try the old "Tupperware Party" trick for getting odors out of those plastic containers
wad up some newspaper and stick it inside, close and let set for a day or so since you say its at school, you may have to try this overnight for several nights, or for a weekend.

Hope 1 of these ideas may help.

Ellie

I agree with Ellie's advice, but I'd just put a cup of vinegar in there until it boils. Seems like that would make more steam and you wouldn't have to worry about the rag or sponge getting too hot. Then use the hot vinegar and some baking soda to clean.

The stain might be a scorch mark, in which case it will never come out.

You're right with baking soda and vinegar, they are the magic ingredients for a lot of things, especially shower stalls I think.
Personally what I like to do with my microwave is this:
I take a microwavable bowl and fill it half way with water, I cut up a lemon or lime ( whatever I have ) and I put that in the water.
I nuke the bowl for a few minutes.
It loosens the dirt so it's much easier to wipe clean and it leaves a fresh clean scent too!

Here is a link that might be useful: everythingschooling

We burned buffalo wings to a crispy wisp of smoke in our microwave---I tried baking soda and lemon, but this was too much...very smoky. The solution was nail polish remover. It wiped off the yellowish residue and removed much of the smell. Work in small areas and wipe off promptly, but it really did the trick! I was told to use acetone, but my polish remover was "acetone-free" and it still worked!

Cut an apple and lay in the microwave. Leave it overnight and it will get rid of most if not all of the odors.