Possession of a school bus for improper purpose
Since you've said "pick your jurisdiction", here. Florida 316.72 makes it a misdemeanor to merely possess such a school bus.
Awkwardly, the statute misses prohibiting the prank itself, but certainly does forbid everything necessary to lead up to the prank: Buying a school bus without obliterating marks and colors; continuing to own it in colors; and using for any other purpose in colors. Take it to Home Depot to buy some plywood, boom, violation.
316.72 Buses simulating school buses in color and insignia; conditions of use.
(1) It shall be unlawful for any person, except a governmental unit or agency operating as provided by law, to use on the public highways of the state any bus of an orange or yellow color known as “school bus chrome,” or any color purporting to resemble the color of a school bus, for any purpose other than to transport persons to and from educational or recreational facilities or institutions or to and from events or activities which are sponsored, financed, or supervised by educational, recreational, religious, or charitable organizations. When said vehicle has ceased to be so used, or is used for the transportation of passengers other than for said purpose, its use shall be unlawful unless and until said bus has been changed from said colors to some other color by repainting and unless and until all signs and insignia which mark or designate it as a school bus have been removed therefrom.
You can weasel that this doesn't strictly prohibit what you are doing. The problem is the chain of custody of the vehicle: it was required to be de-marked the moment it left school bus service.
You will have committed this crime by being in possession, not promptly removing, and maintaining the look of a school bus, with the intent of doing this stunt, even if the actual doing of the stunt does not violate this code.
There are a couple of things that will nail you.
First, when you join the route partially, it means that the first boarding kids will see an empty bus when they don't expect to. They may also notice not all their friends are being picked up. First, you'll have to interact with them about that; which means you are lying already. And second, they're gonna be on their phones telling everyone about it.
Meanwhile, you're early. Children walking to the bus stop, with line-of-sight to same, will see you come and go. Presuming they have missed the bus, they'll run home so their parents can take them. And now the parents are calling the school.
Meanwhile, transit-bus dynamics are working against you, specifically the "bus bunching" effects. Anybody who rides busy transit lines has found themselves on a "follower" bus, picking up few people, making few stops, while getting closer and closer to the bus on the schedule ahead. That "leader" is picking up all the business, and taking more time doing it, while you pick up little, and quickly catch up.
Now you're the leader. Your follower is the legitimate school bus. You're doing most of its business, so it's catching up fast. In no time, it's in sight distance of you, seeing you doing it -- Boom! Call 9-1-1.
You're cursed either way - start later to reduce bus bunching, and you get suspicious kids.
The state will assert that your plan was to diverge before the school, and abduct the children. Your entire logic is that you'll have an airtight defense: that you did in fact drop them off at the school, drove away empty, and all kids are accounted for and unmussed.
You'll never get to the school. Between the proper school bus and the police, all of whom have radio, they will cut you off and box you in, with the mama-bear fervor of protecting their children.
Wrath and fury
If you followed the Mooninite incident of 2007, you know the authorities can be ridiculously unfair. What, in 20/20 hindsight, were obviously advertising street art, was classified by the authorities as a "hoax device" - literally, a fake bomb designed to terrorize. Further, the authorities blamed the ad agency for every single bit of the civil disturbance that was actually caused by the police overreacting - evacuations, road diversions, closed subway lines, thousands of police hours.
So even in the happiest possible case where you let your chargés off at the school without suspicion or incident, and are 5 miles gone before the cops stop you, and are somehow able to keep all this in the civil or misdemeanor realm... They're still gonna go full-metal-stupid on you. They'll charge you with kidnaping, saying that was your intent, but you took them to school because you wimped out. The kids will be none the wiser until the investigators and trauma counselers drag them into rooms and tell them what a terrifying ordeal they suffered. They'll have PTSD for years.
And of course, the families will come after you civilly for the fright you put them through, and the long-term PTSD treatments. Liabilities from this kind of act can't be cleared in bankruptcy, so you'll be paying for the rest of your life.