OP, your post reads like you have had this problem with 2 separate iPads: an original and then a new one. I'm interpreting it to be that you had this problem with BOTH of them. If so, that rules out a LOT of things (defective iPad, iPad battery, charging tech, etc).
We can also completely dismiss a stock software issue or 16.3 bug, else EVERYONE would be sharing this problem too.
So what variables are left?
- The case? If both were used in the same case and that case might step in for charging purposes (battery case or hard/keyboard case with charging passthrough?), perhaps it has some flaw that is quickly draining the battery? If it is a case with only a cut out to the Apple port for charging, rule out the case. You mention Apple's Magic keyboard. Perhaps cut that OUT of the equation to rule it out as contributing culprit?
- The charger(s) you are using? Have you tried a different charger? Perhaps it has failed or barely functions and you are getting only a minimal charge after- presumably- giving it the many hours it needs to charge up to full. Perhaps the charger is defective and actually damaging the iPad? Battery? I once fried an iPhone in a case by only plugging it into a new fast charge charger. Apparently the case could not manage fast charge technology and quickly trashed the iPhone. When you get to school are you connecting it to some charger tech there? Perhaps IT is defective and/or draining instead of charging? Don't use the school connections to rule that out.
- You? While unlikely, you should understand concepts like these do not instantly charge (hours at least). I don't see you making any reference to seeing "100%" in the upper right to confirm full charge. While these are water resistant, does yours end up getting "washed" in water? Etc.
Post #6 "dead as in dead" (not drained): implies SOMETHING is damaging 2 separate iPads. Is it only the drive to school or can you kill iPads without involving the vehicle? Does anyone's iPhone or (their) iPads also die after a comparable ride in your car? If the dead relationship is only with the car ride, what happens in the car ride? Are you using the iPad connected to the car? If so, scrutinize the connecting tech. Perhaps it is frying both iPads? Old car charger? Third party CarPlay connection?
If you are connecting them to anything in your car, try NOT connecting them for the same ride and see what happens. Maybe stop every few miles on the way and check the iPad battery indicator. Get someone to ride with you, keep the iPad on and simply watch the battery indicator as you ride. If it is dropping rapidly, there's something about the car (this seems highly unlikely but you can rule it out with a simple drive).
Are you putting the iPad in the dash window or where it can get pounded by direct sunlight. If they overheat, they will seem dead. I've made mine seem like that by taking it to a pool and letting the sun overheat it. Once it cools back down, it is usually OK again. I've since learned to be sure iDevices are well shaded.
Presumably, you will replace the dead iPad again. My suggestion: do NOT restore existing software or settings to it immediately. In other words, with it set up with ONLY Apple stock apps, get someone else with a different car to give you a ride to school. That rules out anything weird happening in/with your vehicle and any rouge apps/customized settings by you draining the battery (to death).
If that is successful, you then take it on a ride to school in your own car. Do NOT hook it to anything (usual?) in the car. Does it work fine on arrival? This rules out the car and rouge apps or your own settings (because you haven't restored to it yet).
If you do hook it to something in the car, try that on the next drive. Does it work fine on arrival? Does battery indicator on arrival show 100% or towards it? Again, stopping every few miles as you go may offer a key clue if the power reserve is dropping rapidly.
At school, are you keeping it near anything with strong magnetic field? Maybe school involves medical scanners? It is generally placed in a spot where it gets heated? Direct sunlight? Does anyone at school have it out for you and might be able to get to it and do something to it without your knowledge? Does school involve extreme cold? A frozen iPad will seem just as dead as an overheated one.
After you do those, restore your own apps & settings. I suggest on a Saturday when you do not need to head to school. Take it for a drive somewhere else and see how it does. Again, take note of the battery indicator on each open. Anything cause it to rapidly fall too fast?
Basically, if you stack up a step-by-step way of changing ONE variable at a time, you will likely discover the cause.
I highly doubt this is 16.3 bug or there would be an endless number of "me too" posts in this thread and an endless number of "me too" threads about this topic.
I hope this is helpful.
