Yes, because they are already doing something that is illegal. Not being able to play the sport will not stop the student from drinking.

No, because teenagers can be 18 while they go to school. Students can go out to lunch and come back to school intoxicated. This could negatively impact how students preform in school.

Even if it is a stress oriented, being rude isn't the answer. There are other ways to deal with stress.

No, teachers should not have guns. If teaches had guns students may try to get into their desk and take the gun. This may lead to more school shootings.