@op - No parenting failure from what i can see, you have a young independant man in your life who is trying to find his way in the world on his own terms and in his own way, with i will say some good morals.
Give him his space allow him to breath and encourage the values he already shows.
Learning to drive without your knowlwdge - independant
Taking the car to help a friend in need , not joy riding shows strength of character, and morals regarding his view on friends (plus it was a sense of adventure for him i am sure

) and i have no doubt in the back of his mind he did know the risks involved although he would never really admit it to you , as his parent.
The important thing is ,did he manage and succeed in assisting his friend?
The fact that his siblings are doing xyz should never matter in any way, and pointing this out is only really negative reinforcement on him and his self esteem.
I hope you never ever say "look how succesful a or b is, what are you doing with your life etc etc"
major fail if so.
Boarding school was simpler in my day, sneaking out the hostel or sneaking a smoke or drink was hostel "tradition" which was followed up with six of the best if caught, also another hostel tradition. You werent really considered accepted until you had "six of the best" for some or other infraction.
Simpler, less complicated times.
And if your son isnt accademically inclined so what, there are many careers, trades that do not require endless hours of meaningless classroom time in an outdated, old fashioned system that schools rather than educates people.
Good for you, for taking him off the "Kiddie Cocaine", it is the last thing a young independant, developing mind needs.