It would probably be helpful to know what size your bus is, its current GVWR and what you intend to do with it.
Mine's an IH shorty with 25500 GVWR but I doubt it will ever see that much weight (not unless I make a trip to Home Depot for about 4 pallets of concrete blocks and a pallet of bagged cement). I haven't weighed mine but I'm guessing somewhere around 8000 - 10K empty weight. A full length 40' bus will scale out somewhere in the 17K range when empty. Even if yours is rated 25500, that still leaves you around 8K to work with.
Increasing a GVWR is no simple project. Manufacturers spend a *LOT* of time, money, and resources carefully engineering these large vehicles (not just buses, but trucks too) for a specific amount of weight. The frame may need to be beefed up, the springs and spring hangers will need to be upgraded, the axle(s) swapped. The engine, transmission, and cooling system brought up to handle the task. A new Certificate of Origin or other certificate of modification will need to be generated (usually by a professional company) for the new GVWR before the state will accept it. This can be important if, by some chance, you get weighed by the authorities.
All this might make sense if engineering a purpose-built vehicle to take on a new life, but not for a school bus - brand new or otherwise. Buses are built every day to many various specifications for various uses; there's no need to re-engineer one that was already built. This goes doubly so in the used market. You'll spend far more money converting it right than you would simply buying another bus with a higher GVWR in the first place.
Now if you're talking about having a bus with a 33K GVWR, and exceeding that ... it make me curious what you actually have in mind ................