Impair driving usually refers to when a driver is under the influence of drugs or alcohol, making it difficult for them to control their steering which most of the time can usually result in crashes, injuries and sometimes even death. However, impair driving is more than just being under the influence of drugs and alcohol, being distracted by texting or calling, or anything that includes a driver being on your phone, is considered impair driving as you are still unable to control, you're driving if you are not focusing on the road. That is what is what impaired driving truly is, not being able to control your driving whether it be from distractions or being under the influence. 
When it comes to the question of which form of impair driving is dangerous, both texting and drunk driving's are the most common cause of car crashes/accidents or deaths.
As to why these are the most common and dangerous form of impaired driving, when one is texting while driving they are focusing on both the road and their phone, and even if it is just for a short amount of time, the time focusing on their phone can result in them hit a car ahead of them, missing a turn, running off the road, or even hitting a pedestrian. As for drunk driving, the driver is unable to think clearly and make rational decision, and in addition are more prone to emotional decisions and potentially getting anger over little things and acting on said emotional.
While I thankfully do not have any personal experiences in impair driving, from either myself or friends or family. I have seen and heard many stories, that have made reminded me of how dangerous impair driving can be and affected how I drive to ensure I never find myself in that kind of situation. One that I have recently seen is of a lady who was mostly on her phone while driving and while she surprisingly didn't hit anyone, she eventually ran off the side of the road, and got herself stuck, and in addition hit someone's mailbox. Admittedly there have been times where I grab my phone usually to answer a call or switch to a different song, om listening to. However, that video made me realize that even if I think I am being quick and safe, that I could end up like that lady, and since then I avoid touching my phone, and will usually call someone back if I am either at my destination or waiting at a stop light. As for drunk driving I have likely like most people have heard multiple stories of accidents and deaths coming from drunk driving, and while I am of the legal age of drinking, I don't even drink or consume any alcohol mainly due to my fear of getting myself in an accident, and worse, possibly being responsible for another man's life.
Now for the question of how could driving education schools teach this to either new or old drivers to prevent any future accidents caused by impaired driving, I feel the best way would be to have others who have once experienced or were themselves impaired drivers and share their stories. My reason for believing this would be best is that they can go into great detail about how impaired driving caused the accident, like explaining how they exactly lost control and how, and the consequence for it. Compared to instructors who could likely only explained could be scenarios and not go into too much detail. While there is the option of showing videos of these accidents of impaired driving, I still feel that bringing in people is a much better option since one as mentioned earlier the guest could go into greater detail about what happened to help people understand just how dangerous impaired driving is and two video or footage of an accident could be too disturbing for people to watch and wouldn't do a job of explaining the accident as well.
As for what I could personally do to help prevent these kinds of accidents from happening to or from others, I could for one do what I am currently doing write about the dangers of impaired driving and share it with many so they could hopefully understand how dangerous it is or maybe even create videos since those are more likely to be found than articles would be. By finding these articles or videos people could learn more about impaired driving which will hopefully encourage them to be safer while driving to prevent hurting themselves or others. 
Impaired driving is a very serious matter, not only could the driver get themselves severely injured or killed, but they could be responsible for the injury or death of many people. However, these could be easily prevented, by just removing all distractions while driving, and either having a driver to take you home when intoxicated or avoiding drinks especially when planning to drive. However more importantly spreading the dangers of impaired driving, is the best way to help prevent it by having others being aware of it, as there are people in the world you likely don't even know about impaired driving and how dangerous it is.