I think the school is being too cautious as to weight restrictions over all, but perhaps it's because beginning riders have terrible stability in the saddle, which the horse tends to reflexively compensate for by shifting under them, so it would tend to tire the horse faster if the beginner is both awkward and heavier. Some horses will fidget, balk a little, or even panic buck the first time or two that a rider who is much heavier than what the horse is accustomed to mounts the saddle.
It's not unusual to see experienced, heavy riders being carried with relative ease by an average sized, healthy horse, though. The horse will tire a little quicker than if they were carrying a smaller person, obviously.
Is the OP far heavier than 210? Like more than 275? That could be a real problem of too much unstable beginner rider weight for even the most robust horse the school owns. And we don't know about what size or age or condition the schools' horses are. If the school simply cannot afford to purchase another, stronger horse, oh well. Asking the school to buy another horse is a bit much, especially if the would-be riding student is quite heavy, like in excess of 300 lbs. Draft horses can handle very heavy riders, but are usually much taller than regular saddle horses, and falling from that greater height would increase the chance of serious injury, although draft horses by nature are very calm and docile. They are also considerably more expensive to buy, and feed, need larger custom sized tack and saddles, and require larger, more expensive doses of meds and vaccines compared to average horses.
