Welcome to the forum, Andrew!
Smoke and liquid.... I'm thinking you ought to recruit a mechanically knowledgeable person to look at that before you do anything else. A shorted battery can explode.
Different diesel engines have different starting rituals. Some start simply by turning the key. Others need you to heat glow-plugs before you crank it (either with the key, or with a separate button). Some may need you to turn on the fuel supply by hand (kind'a like old motorcycles lol).
You might want to start reading -- reading all you can find on diesel engines, school buses, and your particular engine model.
And air brakes. You say your air brakes pressurize. Perhaps there is some pressure left from the last time it ran, but the compressor is driven by the engine, so the engine must run to build pressure. Much to learn here, my friend.
You say you charged the batteries to 100%. How do you know that? Voltmeter? Hydrometer?
It's awful easy to make assumptions, but things can go wrong when we don't actually KNOW what we are doing.
I don't mean to pick on you! I'm trying to educate you, as I needed to be educated on all this Stuff in my day. (I'm 64, and I've been both an auto mechanic and a truck driver. Kept learning the whole way.)
25 cents says you overcharged the batteries by leaving the charger on all day. That could explain liquid and smoke. How old is the charger? Is it a modern one that shuts itself off when the battery is full? If not.... Old style chargers just keep pumping until "the balloon pops".
And yes, and Carytowncat says, one thing you can do now is take the batteries to an auto parts store for inspection and perhaps testing.
Once the battery situation is sorted out, you may or may not have a cranking problem. Let's take one step at the time.
And smile. Always smile. Specially when you are learning New Stuff.