My two pence worth,
We operate a S76 daily out of two stud farms with up to 80 racehorses in each. When they are in the fields they form a large herd with an alpha (male or female) who controls their behaviour. If he/she likes helicopters he/she will be calm and so will the rest of the herd and quite often they will stand there and just look at us.
It also helps having a noisy helicopter (I am told by our racing manager) as if they can hear it coming they look for it, assess the threat and reactive accordingly. The problem comes if the threat pops up from nowhere and they have no time to assess so flight or fight kicks in and they are spooked. We have horses that throw the jockeys if a piece of paper blows in front of them, then do it again 10 minutes later when the same piece of paper blows past again but will stand and look at us landing 10 metres away.
If you are setting up a HLS at a riding school you are going to have a real mix of horses, all with different characters. Most of the time they are not going to form herds so will do there own thing or react to what everyone else is doing. Add to the mix inexperienced handlers and riders and you can have a real bag of potential problems.
You also have to look at the helicopters and pilots that are going to be using the HLS. Most will be professional and considerate but this industry is blighted with people that know best and like to show people what they have achieved in life and how good they are. They are generally arrogant and don't take advice readily. It's these few who will cause the problem. As they say "You can't make things idiotproof because idiots are very ingenius."
Personally I wouldn't set up a HLS at a riding school for the above reasons.
If you are, don't site the HLS adjacent to the school put it in a field away from the school where the horses are in fields so have room to see you coming and have room to move away from the threat or react a little. Site it downwind from school for the prevailing wind direction.
I would vet the pilots and have a strict set of operating procedures and get lots of insurance!!
Remember what you do will affect the whole industry.
Good Luck.