FWIW, I've had very inexperienced riders trail ride our horses in a western curb without a problem.
English riding involves constant contact, which would make it much harder to use a curb properly than the western approach, but folks ought to stop worrying about "leverage".
It isn't pressure that hardens a horse's mouth, but the failure to release pressure at the right time. That was a point I first encountered reading Chamberlin, and Hilary Clayton makes the same point in this article:
On contact, rein tension, and the myth of lightness
By Hilary M. Clayton, BVMS, PhD, DACVSMR, MRCVS
...As dressage riders and trainers, we value the quality of lightness; but when I measure the amount of tension associated with a contact that feels correct to an experienced rider, it oscillates from about one pound to five pounds. Although five pounds sounds like a lot of tension to hold in your hand, it really doesn’t feel like a lot of weight in a dynamic situation. Some dressage texts lead us to believe that fully trained horses should take only a few ounces of contact with the rein. However, this advice is highly
subjective and is not supported by actual measurements. Moreover, if rein tension is a consequence of the natural movements of the horse’s relaxed neck, then the mechanics of the motion dictate the amount of tension...
...It is difficult for a rider to correctly assess the amount of tension in the reins when the contact is dynamic and tension is constantly changing. Our goal as riders should be to offer our horses a consistent and predictable contact that allows them to seek the bit confidently and, in so doing, to use their entire bodies correctly..."
The pressure used by good dressage riders turned out to be much higher than Clayton had expected, but it still works without the horse getting a hard mouth because it is TIMING that is critical, not PSI. If you have contact that follows the horse, and adjusts as needed to how the horse responds, then PSI is not very important.
Also, the amount of mechanical advantage provided by most curb bits is not overwhelming - typically around 3:1, so someone applying 2 lbs of pressure to the reins gives 6 lbs to the mouth...and it is not that hard to ride with 2lbs of pressure or less. That is true of western and English curbs. It depends on shank ratios, and that ratio is normally around 2:1 lower:upper regardless of style (altho there are exceptions...I've seen 5:1).
A rider who balances with the reins or who snatches on the reins should not use a curb bit. I'm not sure they ought to use a snaffle, either
. But since the instructor picked the bit and will be watching when it is first used, the odds of anything cruel happening is miniscule. Unlike any of us, the instructor has seen both the horse & the rider, and will be doing so while the bit is first used.