How Difficult is 31b Military Police AIT?

Good rule of thumb: Better people than you have completed the course... so have lesser. Study hard, don't do dumb shit, and practice your Army Values and there is no telling where you can go.

How difficult is XXXXX course is going to be the most subjective question you could ask. Better to first search and see if this following question has been asked: "what should I do to prepare for XX AIT/School?".
One free piece of advice: military schools don't require a degree.

They are easy and usually just go/no go. Law is usually open book so take a crap load of notes (plus it’ll keep you occupied so you don’t fall asleep). If you fail anything they give you plenty of opportunities to retest until you pass.