are you allowed to leave the campus in boarding school?

It depends on the boarding school, but generally you can only leave during the class day if you are signed out by an adult and you aren't going to miss any obligations (i.e., class). Your parents will be asked to specify what adults should be allowed to sign you out, whether it be only family members, or if they want to include the parents of other students and faculty members, or all adults. In boarding school terms, "adult" usually only applies to people 21 or older, so if someone is only 18 that doesn't count.

Weekends may not be the only time you are allowed off campus, though. At my boarding school, as you get older you are allowed to go more often during the week (in the afternoons though not during the class day) and seniors are allowed to go during the class day during lunch, free periods, etc. Your school may do the same thing.

Just don't sneak off campus when it isn't allowed...schools tend to treat that as a major offense, on par with drinking and such.

It depends upon the type of boarding school and the rules they have in place. For instance, the military-style boarding school will probably have more restrictive rules. A school for under performing students or students with drug problems will probably also have stricter rules. Also, 1st year students will probably have more limitations than older students. Why not look at the school's website and see if they have a student manual posted.

Each school will have its own rules for situations like this. It will be necessary to familiarise yourself with all the particular school's rules and regulations when you get there.