Are you teachers explaining HOW to have a horse first 'accept the bit' and then gradually go 'on the bit' (with a degree of flexion suiting his age and training). Are they explain the effect of your aids. Remember 'chewing the reins from the hand' aka forward down out aka stretching is a TEST of proper training not something that comes first. Long and low can mean low flat neck or a closed postured or dangling the reins, it is rather meaningless statement imho.
So, how does the rider present the bit to the horse? First, is there a straight line from elbow to horse's mouth (no matter HOW high they are), so that the bit does not act on the bars, but on the corners of the mouth. Secondly, the horse must be ridden with a smidge of lateral flexibility (for this carrying the inside hand a smidge higher can help form the basis (for working into the outside rein). All this should be being explained to you by teachers. It is NOT about making the horse low or closed (remember the mouth should be about the level of the point of the hip), it is about MAINTAINING a light connection and lateral flexibility (hence riding of MANY circles/curved lines/serpentines/etc) and PULSING the leg aids.
For what it is worth, 'resistances' in the horse are ALWAYS we riders. (Maybe 1% is something else). We act, the horse reacts. If the horse misunderstands out tact/timing/alignment/etc it is UP to us to understand why and change our tact/timing/duraction/etc. That is why riding methodologies are so necessary in training, and something that must be addressed in every ride.
A horse just coming 5 is a teenager. Testing the limits/learning to understand. If the rider becomes the reaction to his actions instead of visa versa then you will always be chasing the escapes.
Horses 'raise their backs' when they work into the hand (in a lightly open posture with active hind legs) with a mobile jaw and wiling keep the connection, then the half halts can create more more folded hind leg joints. Create too much (precipitous) longitudinal flexion and the connection with the hind legs is lost, and the horse struggles against the pain on the bars and the effect of the hand.
Think more about riding 'in position' (can you see the inside eyelashes), and keeping the straight line to the mouth (no resting the hands on the withers), pulse the (leg) aids.