These have their own specialist Highway Code signs in and around their campuses. Mostly these restrict speed but others control how and when students fly (never when drunk, though this is a rule more often breached than kept and is still the biggest cause of flying accidents in the Kingdom).
Drugs are not an issue here. Given everyone can cause and experience illusions with a handy spell, why take an artificial substance to create something that would be inferior to what can be magically produced? Alcohol though is one of the most popular imports from Earth and not just for students.
There are also signs banning magical fighting on the public highway (there is nothing covering side alleys, so presumably it is okay to fight there). Part of the Highway Code also covers magical etiquette on the grounds if you follow this, you will obey the vast majority of the Code anyway.
Students have the largest chapter on how to behave with their powers, wands etc as it is considered they need telling. One of their specialist road signs is the specific banning of magical showing off. People get tired of it. Too many innocent magical beings have got in the way accidentally and been turned into a species they weren't originally born into and the fairies especially became tired of rectifying this. The witches loved it. To have so many new toads, newts etc on tap for use in spells had been wonderful. They didn't care then these things weren't originally amphibians. They still don't care. (So don't get turned into an amphibian in the magical world - you're unlikely to have a long and happy life).